Patent Publication Number: US-2023132581-A1

Title: Scroll compressor

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a U.S. national stage application of International Patent Application No. PCT/JP2020/018962 filed on May 12, 2020, the disclosure of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a scroll compressor configured to compress a working medium. 
     BACKGROUND 
     Hitherto known scroll compressors are each configured to compress a working medium in a plurality of compression chambers defined between scroll wraps of a fixed scroll and an orbiting scroll that are made to mesh with each other. The working medium is compressed when the orbiting scroll undergoes an orbital motion about the fixed scroll. Such a scroll compressor is disclosed by Patent Literature 1, in which a scroll start portion, also regarded as a scroll center portion, of a scroll wrap has a tiered shape whose thickness is reduced from the base toward the tip. 
     PATENT LITERATURE 
     
         
         Patent Literature 1: International Publication No. 2015/040720 
       
    
     According to Patent Literature 1, among a plurality of compression chambers, an innermost compression chamber and a second compression chamber on the radially outer side of the innermost compression chamber are made to communicate to each other in a graded manner by employing the tiered scroll start portion. Thus, the stress generated at the base of the scroll start portion is reduced. However, discussions on some issues have not been made specifically, including which region of the scroll start portion is to be shaped in tiers. Therefore, whether the stress is reduced satisfactorily is unclear. 
     Moreover, if the fixed scroll and the orbiting scroll are made of respective materials having different coefficients of linear expansion, the difference in the amount of thermal expansion occurring with a temperature rise during operation may generate a great stress at the base of the scroll start portion. Such an issue is not discussed in Patent Literature 1. 
     SUMMARY 
     The present disclosure is to solve at least one of the above problems and provides a scroll compressor in which the stress generated at the base of the scroll start portion is small. 
     A scroll compressor according to a first embodiment of the present disclosure is configured to compress a working medium in a plurality of compression chambers defined between an orbital scroll wrap of an orbiting scroll and a fixed scroll wrap of a fixed scroll that are made to mesh with each other. The working medium is compressed when the orbiting scroll driven through a main shaft undergoes an orbital motion about the fixed scroll. The orbital scroll wrap and the fixed scroll wrap include respective scroll start portions each having a bulbous shape defined by connecting an involute start point of an outer-surface involute curve and an involute start point of an inner-surface involute curve to each other with a plurality of arcs. At least one of the scroll start portions has a tiered shape in which an n number (where n≥2) of tiers each having the bulbous shape are stacked in an axial direction of the main shaft. Letting involute-start-point angles of the outer-surface involute curves in the respective tiers of the scroll start portion having the tiered shape be φos(1), φos(2), φos(3), . . . , and φos(n) in order from a tip toward a base of the scroll start portion, the following relationships are satisfied: φos(1)&gt;φos(2)&gt;φos(3)&gt; . . . &gt;φos(n); and 0.3π&lt;φos(1)-φos(n)&lt;0.7π. 
     A scroll compressor according to a second embodiment of the present disclosure is configured to compress a working medium in a plurality of compression chambers defined between an orbital scroll wrap of an orbiting scroll and a fixed scroll wrap of a fixed scroll that are made to mesh with each other. The working medium is compressed when the orbiting scroll driven through a main shaft undergoes an orbital motion about the fixed scroll. The orbital scroll wrap and the fixed scroll wrap include respective scroll start portions each having a bulbous shape defined by connecting an involute start point of an outer-surface involute curve and an involute start point of an inner-surface involute curve to each other with a plurality of arcs. At least one of the scroll start portions has a tiered shape in which an n number (where n≥2) of tiers each having the bulbous shape are stacked in an axial direction of the main shaft. The orbiting scroll and the fixed scroll are made of respective materials having different coefficients of linear expansion. Letting the tiers of the scroll start portion be defined as a first tier, a second tier, . . . , and an n-th tier in order from a tip toward a base of the scroll start portion; and a situation where the orbital scroll wrap and the fixed scroll wrap go out of contact with each other in the n-th tier of the scroll start portion and, among the compression chambers, two compression chambers that are not made to communicate with each other before the scroll wraps go out of contact with each other are made to communicate with each other be expressed as the n-th tier is opened, a relief is provided in an outer surface of the orbital scroll wrap or in the fixed scroll wrap such that while the orbiting scroll is undergoing the orbital motion from a crank angle at which the first tier is opened to a crank angle at which the n-th tier is opened, the outer-surface involute of the scroll wrap of the scroll that is made of the material having the greater coefficient of linear expansion and the inner-surface involute of the scroll wrap of the scroll that is made of the material having the smaller coefficient of linear expansion are out of contact with each other at least at an outermost one of a plurality of contact points where the two involutes are to come into contact with each other. 
     In the scroll compressor according to the first embodiment of the present disclosure, since the scroll start portion has the tiered shape, among the plurality of compression chambers, the innermost compression chamber and the second compression chamber on the radially outer side of the innermost compression chamber are made to communicate with each other in a graded manner. Thus, the stress generated at the base of the scroll start portion is reduced. Furthermore, satisfying the relationship of 0.3π&lt;φos(1)-φos(n)&lt;0.7π brings a satisfactory degree of strength improvement for the scroll start portion. 
     In the scroll compressor according to the second embodiment of the present disclosure, the scroll start portion of the scroll wrap made of the material having the greater coefficient of linear expansion is kept supported by the lateral face of the scroll wrap of the scroll made of the material having the smaller coefficient of linear expansion until the pressure is completely equalized between two of the compression chambers, namely the innermost compression chamber and the second compression chamber that are not made to communicate with each other before the scroll wraps go out of contact with each other. Such a configuration suppresses the generation of a great stress at the base of the scroll start portion of the scroll wrap made of the material having the greater coefficient of linear expansion. Furthermore, the scroll start portion of the scroll wrap made of the material having the smaller coefficient of linear expansion is designed such that, during operation, the gaps from the other scroll wrap made of the material having the greater coefficient of linear expansion become smaller than in a case where no relief is provided. Such a configuration suppresses the generation of a great stress at the base of the scroll start portion. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    schematically illustrates a longitudinal section of a scroll compressor according to Embodiment 1. 
         FIG.  2    includes diagrams illustrating a scrolling motion undergone by the scroll compressor according to Embodiment 1 in a compression process. 
         FIG.  3    schematically illustrates a lateral section of a compression portion included in the scroll compressor according to Embodiment 1. 
         FIG.  4    is an enlarged perspective view of a scroll start portion of a fixed scroll included in the scroll compressor according to Embodiment 1. 
         FIG.  5    is an enlarged perspective view of a scroll start portion of an orbiting scroll included in the scroll compressor according to Embodiment 1. 
         FIG.  6    is a further enlarged plan view of the scroll start portion of the fixed scroll included in the scroll compressor according to Embodiment 1. 
         FIG.  7    includes enlarged plan views of the scroll start portions of the fixed scroll and the orbiting scroll included in the scroll compressor according to Embodiment 1. 
         FIG.  8    illustrates how the pressure acts on a scroll start portion according to a comparative example at the start of pressure equalization. 
         FIG.  9    illustrates how the pressure acts on the scroll start portion of the scroll compressor according to Embodiment 1 at the start of pressure equalization. 
         FIG.  10    illustrates how the pressure acts on the scroll start portion of the scroll compressor according to Embodiment 1 after the completion of pressure equalization. 
         FIG.  11    illustrates the change in the stress generated at the base of the scroll start portion versus the change in the crank angle in the scroll compressor according to Embodiment 1. 
         FIG.  12    is an enlargement of the scroll start portion of the scroll compressor according to Embodiment 1 in a case where φos(1)-φos(n) is 0.2π. 
         FIG.  13    is an enlargement of the scroll start portion of the scroll compressor according to Embodiment 1 in a case where φos(1)-φos(n) is 0.5π. 
         FIG.  14    includes diagrams illustrating the relationship between the direction in which a load acts on the scroll start portion in a known configuration taken as the comparative example and the thickness of the scroll start portion that receives the load. 
         FIG.  15    includes diagrams illustrating the relationship between the direction in which a load acts on the scroll start portion of the scroll compressor according to Embodiment 1 and the thickness of the scroll start portion that receives the load in the case where φos(1)-φos(n) is 0.2π. 
         FIG.  16    includes diagrams illustrating the relationship between the direction in which a load acts on the scroll start portion of the scroll compressor according to Embodiment 1 and the thickness of the scroll start portion that receives the load in the case where φos(1)-φos(n) is 0.5π. 
         FIG.  17    includes diagrams illustrating the relationship between the direction in which a load acts on the scroll start portion of the scroll compressor according to Embodiment 1 and the thickness of the scroll start portion that receives the load in the case where φos(1)-φos(n) is 0.7π. 
         FIG.  18    illustrates the result of a strength analysis conducted on the scroll start portion of the scroll compressor according to Embodiment 1. 
         FIG.  19    is a schematic enlargement of a longitudinal section of the scroll start portion of the scroll compressor according to Embodiment 1 and peripheries thereof. 
         FIG.  20    schematically illustrates a lateral section of a compression unit of a scroll compressor according to Embodiment 2. 
         FIG.  21    is a perspective view of one of reliefs provided in the scroll compressor according to Embodiment 2. 
         FIG.  22    schematically illustrates a lateral section of a compression unit according to a comparative example, with an orbiting scroll being made to uneccentrically mesh with a fixed scroll, and also illustrates gaps produced between an orbital-outer-surface involute and a fixed-inner-surface involute at room temperature. 
         FIG.  23    schematically illustrates a lateral section of the compression unit according to the comparative example, with the orbiting scroll being made to uneccentrically mesh with the fixed scroll, and also illustrates gaps produced between the orbital-outer-surface involute and the fixed-inner-surface involute during operation. 
         FIG.  24    illustrates the gap sizes, δ 0 , of the respective gaps produced at room temperature in the compression unit according to the comparative example, with the orbiting scroll being made to eccentrically mesh with the fixed scroll. 
         FIG.  25    illustrates the changes, δa, in the sizes of the respective gaps produced in the compression unit according to the comparative example, between the gap sizes at room temperature and the gap sizes during operation. 
         FIG.  26    illustrates the gap sizes, δs, of the respective gaps produced during operation in the compression unit according to the comparative example. 
         FIG.  27    illustrates the gap sizes δ 0  of the respective gaps produced at room temperature in the compression unit of the scroll compressor according to Embodiment 2, with the orbiting scroll being made to eccentrically mesh with the fixed scroll. 
         FIG.  28    illustrates the effect produced by the reliefs provided in the compression unit of the scroll compressor according to Embodiment 2, that is, the differences, δb, between the gap sizes at room temperature in the comparative example and the gap sizes during operation in Embodiment 2. 
         FIG.  29    illustrates the gap sizes δs of the respective gaps produced during operation in the compression unit of the scroll compressor according to Embodiment 2. 
         FIG.  30    schematically illustrates a lateral section of a compression unit of a scroll compressor according to Embodiment 3. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure will now be described with reference to the drawings. Note that the present disclosure is not limited by the following embodiments. The same reference signs provided in the drawings denote the same or equivalent elements, which applies throughout this specification. The forms of the individual elements described throughout the entirety of this specification are only exemplary and are not limited thereto. 
     Embodiment 1 
       FIG.  1    schematically illustrates a longitudinal section of a scroll compressor according to Embodiment 1. 
     The scroll compressor,  1 , will now be described with reference to  FIG.  1   . The scroll compressor  1  is configured to suction a working medium such as refrigerant circulating through a refrigerant circuit; compress the working medium into a high-temperature, high-pressure medium; and discharge the high-temperature, high-pressure medium. The scroll compressor  1  includes a compression unit  5 , a motor  4 , and other relevant elements. The motor  4  drives the compression unit  5  through a main shaft  7 . The foregoing elements are housed in a shell  2 , which serves as an outer shell. As illustrated in  FIG.  1   , the compression unit  5  is located in an upper part and the motor  4  is located in a lower part in the shell  2 . The bottom of the shell  2  forms an oil sump  3   a , where lubricant is stored. 
     The shell  2  further houses a frame  6  and a sub frame  20 , which are located across the motor  4  from each other. The frame  6  is located above the motor  4  and between the motor  4  and the compression unit  5 . The sub frame  20  is located below the motor  4 . The frame  6  and the sub frame  20  are fixed to the inner peripheral face of the shell  2  by a method such as shrink fitting or welding. The frame  6  is provided in a central part thereof with a main bearing  8   a . The sub frame  20  is provided in a central part thereof with a counterbearing  8   b . The counterbearing  8   b  is, for example, a ball bearing and is press-fitted to the sub frame  20 . The main bearing  8   a  and the counterbearing  8   b  support the main shaft  7  while allowing the main shaft  7  to rotate. 
     The sub frame  20  is provided with a displacement oil pump  3 . A pump shaft for transmitting a turning force to the oil pump  3  is integrated with the main shaft  7 . The main shaft  7  has an oil hole  7   b . The oil hole  7   b  extends through the center of the main shaft  7  from the lower end of the pump shaft to the upper end of the main shaft  7 . The lower end of the oil hole  7   b  is connected to the oil pump  3 . 
     The shell  2  includes three parts: an upper shell  2   a , a middle shell  2   b , and a lower shell  2   c . The shell  2  is provided with a suction pipe  11 , for suctioning the refrigerant; and a discharge pipe  12 , for discharging the refrigerant. The refrigerant suctioned into the shell  2  through the suction pipe  11  flows through a suction port  6   a , provided in the frame  6 , and is suctioned into compression chambers  5   a , provided in the compression unit  5  and to be described separately below. 
     The compression unit  5  is configured to compress the refrigerant suctioned thereinto through the suction pipe  11  and to discharge the compressed refrigerant to a high-pressure section provided in an upper part of the shell  2 . The compression unit  5  includes a fixed scroll  30 , an orbiting scroll  40 , an Oldham ring  15 , and other relevant elements. The Oldham ring  15  is configured to prevent the orbiting scroll  40  from spinning on its own axis while the orbiting scroll  40  is undergoing an eccentric circular motion (swirling motion). 
     The fixed scroll  30  is located on the upper side and is fixed to the shell  2  with the frame  6  interposed therebetween. The orbiting scroll  40  is located on the lower side and is supported by the main shaft  7  while being allowed to swirl. 
     The fixed scroll  30  includes a fixed base plate  30   a  and a fixed scroll wrap  30   b . The fixed scroll wrap  30   b  has a scroll shape and is provided on one face of the fixed base plate  30   a . The orbiting scroll  40  includes an orbital base plate  40   a  and an orbital scroll wrap  40   b . The orbital scroll wrap  40   b  has a scroll shape and is provided on one face of the orbital base plate  40   a . The fixed scroll wrap  30   b  and the orbital scroll wrap  40   b  are each shaped in conformity with, for example, an involute curve. The fixed scroll  30  and the orbiting scroll  40  are arranged in the shell  2  such that the fixed scroll wrap  30   b  and the orbital scroll wrap  40   b  are in mesh with each other. Between the fixed scroll wrap  30   b  and the orbital scroll wrap  40   b  are defined a plurality of compression chambers  5   a . While the main shaft  7  is rotating, the capacities of the compression chambers  5   a  decrease in the radial direction from the outer side toward the inner side. 
     The fixed scroll  30  has in a central part thereof a discharge port  30   f , through which the refrigerant having a high pressure by being compressed is discharged. On the exit side of the discharge port  30   f  is provided a discharge chamber  13 . The discharge chamber  13  is provided at the outlet thereof with a discharge valve  13   a , which has a reed-valve structure. Above the discharge chamber  13  is provided a muffler  14 . The muffler  14  suppresses the pulsation of the working medium discharged from the discharge chamber  13 . 
     The orbital base plate  40   a  of the orbiting scroll  40  has an orbital bearing  40   f . The orbital bearing  40   f  is provided in a central part of the other face of the orbital base plate  40   a  that is opposite the face having the orbital scroll wrap  40   b . The orbital bearing  40   f  has a bore, where a slider  9  to be described below is rotatably supported. The center axis of the orbital bearing  40   f  is parallel to the center axis of the main shaft  7 . 
     The Oldham ring  15  is located between the orbiting scroll  40  and the frame  6 . The Oldham ring  15  includes a ring portion, a pair of Oldham keys provided on the upper face of the ring portion, and another pair of Oldham keys provided on the lower face of the ring portion. The Oldham keys on the upper face are fitted in respective key grooves provided in the orbiting scroll  40  and are slidable in one direction. The Oldham keys on the lower face are fitted in respective key grooves provided in the frame  6  and are slidable in a direction intersecting the one direction. Such a configuration allows the orbiting scroll  40  to undergo an orbital motion without spinning on its own axis. 
     The motor  4  includes a stator  4   b  and a rotor  4   a . The stator  4   b  is fixed to the inner periphery of the shell  2 . The rotor  4   a  is located on the inner side of the stator  4   b . The rotor  4   a  is fixed to the main shaft  7  by shrink fitting or any other method. The stator  4   b  receives electric power through a power terminal  21 , which is provided on the shell  2 . When electric power is supplied to the stator  4   b , the rotor  4   a  rotates together with the main shaft  7 . 
     The main shaft  7  includes at the upper end thereof an eccentric shaft portion  7   a . The eccentric shaft portion  7   a  is shifted from the center axis of the main shaft  7  in a predetermined direction of eccentricity. The eccentric shaft portion  7   a  is slidably fitted in the slider  9  to be described below. 
     The slider  9  serves as a variable crank mechanism that makes the radius of the orbital motion of the orbiting scroll  40  vary along the lateral face of the fixed scroll wrap  30   b  of the fixed scroll  30 . The variable crank mechanism keeps the lateral face of the fixed scroll wrap  30   b  and the lateral face of the orbital scroll wrap  40   b  in contact with each other while the orbiting scroll  40  is undergoing the orbital motion. 
     An overall operation of the scroll compressor  1  will now be described briefly. When electric power is supplied to the stator  4   b , the rotor  4   a  rotates. A rotational driving force generated by the rotor  4   a  is transmitted to the orbiting scroll  40  through the main shaft  7 , the eccentric shaft portion  7   a , and the slider  9 . The orbiting scroll  40  having received the rotational driving force undergoes a swirling motion about the fixed scroll  30  while being prevented by the Oldham ring  15  from undergoing a spinning motion. 
     With the swirling motion of the orbiting scroll  40 , low-pressure gas refrigerant is suctioned into the shell  2  through the suction pipe  11 , is drawn into the compression chambers  5   a  through the suction port  6   a  provided in the frame  6 , and is compressed in the compression chambers  5   a . The compressed gas refrigerant now having a high pressure is discharged to the discharge chamber  13  through the discharge port  30   f . The high-pressure gas refrigerant in the discharge chamber  13  pushes the discharge valve  13   a  upward, flows into the space in the muffler  14 , and is discharged to the inside of the shell  2  through a discharge hole provided in the muffler  14 . Then, the refrigerant is discharged to the outside of the scroll compressor  1  through the discharge pipe  12 . 
       FIG.  2    includes diagrams illustrating a scrolling motion undergone by the scroll compressor according to Embodiment 1 in the compression process. Referring to  FIG.  2   , the compression process will now be described. Note that the detailed shape of a scroll center portion, also regarded as a scroll start portion, will be described separately below. Among the plurality of compression chambers  5   a  illustrated in  FIG.  2   , the innermost one is referred to as innermost compression chamber  5   a   1 , the outermost ones are each referred to as outermost compression chamber  5   a   3 , and the ones between the innermost compression chamber  5   a   1  and the outermost compression chambers  5   a   3  are each referred to as second compression chamber  5   a   2 . 
     In  FIG.  2   , diagram (a) illustrates a position of the orbiting scroll  40  that is in mesh with the fixed scroll  30  where the outermost compression chambers  5   a   3  are defined at the completion of suction. Diagram (b) illustrates a position of the orbiting scroll  40  that has undergone the orbital motion by 90 degrees from the position of suction completion illustrated in diagram (a). Diagram (c) illustrates a position of the orbiting scroll  40  that has undergone the orbital motion by 180 degrees from the position of suction completion illustrated in diagram (a). Diagram (d) illustrates a position of the orbiting scroll  40  that has undergone the orbital motion by 270 degrees from the position of suction completion illustrated in diagram (a). The orbiting scroll  40  undergoes a swirling motion by taking the positions (a), (b), (c), (d), and (a) in that order. That is, the orbiting scroll  40  undergoes an orbital motion without undergoing a spinning motion. In such a motion, the capacities of the compression chambers  5   a  decrease in order of the outermost compression chambers  5   a   3 , the second compression chambers  5   a   2 , and the innermost compression chamber  5   a   1 . Therefore, the suctioned refrigerant is compressed while being sent toward the center, and is discharged from the innermost compression chamber  5   a   1  to the outside of the scroll compressor  1  through the discharge port  30   f  provided in the fixed scroll  30 . 
     The fixed scroll  30  and the orbiting scroll  40  each include a scroll start portion, which is also regarded as a scroll center portion of the scroll wrap. The scroll start portion has a so-called bulbous shape defined by connecting the involute start points of respective involute curves forming the inner surface and the outer surface of the scroll start portion to each other with two arcs of a small circle and a large circle. The scroll start portion according to Embodiment 1 has a tiered shape in which a plurality of tiers each having the bulbous shape are stacked in the axial direction of the main shaft  7 . Hereinafter, such a shape of the scroll start portion is also referred to as tiered bulbous shape. 
       FIG.  3    schematically illustrates a lateral section of the compressor mechanical unit included in the scroll compressor according to Embodiment 1.  FIG.  4    is an enlarged perspective view of the scroll start portion of the fixed scroll included in the scroll compressor according to Embodiment 1.  FIG.  5    is an enlarged perspective view of the scroll start portion of the orbiting scroll included in the scroll compressor according to Embodiment 1. 
     As illustrated in  FIGS.  3  and  4   , the scroll start portion,  30   e , of the fixed scroll wrap  30   b  of the fixed scroll  30  has, for example, three tiers: a first tier  30   e   1 , a second tier  30   e   2 , and a third tier  30   e   3  in that order from the tip thereof. The scroll start portion  30   e  of the fixed scroll wrap  30   b  of the fixed scroll  30  have small arc parts, whose positions are sequentially shifted in a direction toward the scroll-starting end and in order in a direction from the tip (the upper side in the drawing) toward the base (the lower side in the drawing). While the drawings illustrate a case of a three-tiered structure, the number of tiers only needs to be n (where n≥2). That is, the scroll start portion  30   e  of the fixed scroll wrap  30   b  only needs to have a tiered shape in which an n number (where n≥2) of tiers each having the bulbous shape are stacked in the axial direction. 
     Among the small arc parts, the one that is nearest to the tip (the one in the first tier) is defined as small arc part  301 , another one that is closer to the base than the first one (the one in the second tier) is defined as small arc part  301   a , and the one that is nearest to the base (the one in the third tier) is defined as small arc part  301   b . The small arc part  301   a  in the second tier is shifted from the small arc part  301  in the first tier in the direction toward the scroll-starting end. The small arc part  301   b  in the third tier is shifted from the small arc part  301   a  in the second tier in the direction toward the scroll-starting end. The radius of the large circle defining the scroll start portion  30   e  of the fixed scroll wrap  30   b  is the same for all of the first tier, the second tier, and the third tier. That is, large arc parts in the respective tiers of the scroll start portion  30   e  form a shared large arc part  302 . 
     In such a configuration, the fixed scroll  30  comes into contact with the inner surface of the scroll wrap of the orbiting scroll  40  with different timings in the first tier, the second tier, and the third tier in that order. 
     As illustrated in  FIGS.  3  and  5   , as with the case of the fixed scroll  30 , the scroll start portion,  40   e , of the orbital scroll wrap  40   b  of the orbiting scroll  40  has, for example, three tiers: a first tier  40   e   1 , a second tier  40   e   2 , and a third tier  40   e   3  in that order from the tip thereof. The scroll start portion  40   e  of the orbital scroll wrap  40   b  of the orbiting scroll  40  have small arc parts, whose positions are sequentially shifted in a direction toward the start of the scroll and in order in a direction from the tip (the upper side in the drawing) toward the base (the lower side in the drawing). While the drawings illustrate a case of a three-tiered structure, the number of tiers only needs to be n (where n≥2). That is, the scroll start portion  40   e  of the orbital scroll wrap  40   b  only needs to have a tiered shape in which an n number (where n≥2) of tiers each having the bulbous shape are stacked in the axial direction. 
     Among the small arc parts, the one that is nearest to the tip (the one in the first tier) is defined as small arc part  401 , another one that is closer to the base than the first one (the one in the second tier) is defined as small arc part  401   a , and the one that is nearest to the base (the one in the third tier) is defined as small arc part  401   b . The small arc part  401   a  in the second tier is shifted from the small arc part  401  in the first tier in the direction toward the scroll-starting end. The small arc part  401   b  in the third tier is shifted from the small arc part  401   a  in the second tier in the direction toward the scroll-starting end. 
     The scroll start portion  40   e  of the orbital scroll wrap  40   b  of the orbiting scroll  40  is defined by large circles with respective radii. The first tier has a large arc part  402 , which is defined by a large circle with the greatest radius. The second tier has a large arc part  402   a , which is defined by a large circle with a radius smaller than the radius of the large circle defining the large arc part  402 . The third tier has a large arc part  402   b , which is defined by a large circle with a radius smaller than the radius of the large circle defining the large arc part  402   a . In Embodiment 1, the involute-start-point angle of the inner-surface involute curve of the orbiting scroll  40  is the same for all of the first tier, the second tier, and the third tier. Accordingly, the radii of the large circles in the respective tiers of the orbiting scroll  40  vary with the radii of the small circles. 
     In such a configuration, the orbiting scroll  40  comes into contact with the inner surface of the scroll wrap of the fixed scroll  30  with different timings in the first tier, the second tier, and the third tier in that order. Note that when the tiers in the scroll start portion  30   e  of the fixed scroll wrap  30   b  and the tiers in the scroll start portion  40   e  of the orbital scroll wrap  40   b  do not need to be distinguished from each other, they are also simply referred to as the first tier, the second tier, and the third tier with no reference signs. 
     The first tier, the second tier, and the third tier of the fixed scroll  30  are defined by small circles having the same radius and large circles having the same radius, whereas the first tier, the second tier, and the third tier of the orbiting scroll  40  are defined by small circles having different radii and large circles having different radii. Specifically, the small circle defining the small arc part  401  in the first tier has the smallest radius, the small circle defining the small arc part  401   a  in the second tier has a greater radius than the small circle defining the small arc part  401 , and the small circle defining the small arc part  401   b  in the third tier has a greater radius than the small circle defining the small arc part  401   a . Conversely, the large circle defining the large arc part  402  in the first tier has the greatest radius, the large circle defining the large arc part  402   a  in the second tier has a smaller radius than the large circle defining the large arc part  402 , and the large circle defining the large arc part  402   b  in the third tier has a smaller radius than the large circle defining the large arc part  402   a . In Embodiment 1, the involute-start-point angle of the inner-surface involute curve of the orbiting scroll  40  is the same for all of the first tier, the second tier, and the third tier. Accordingly, the radii of the large circles in the respective tiers of the orbiting scroll  40  vary with the radii of the small circles. 
       FIG.  6    is a further enlarged plan view of the scroll start portion of the fixed scroll included in the scroll compressor according to Embodiment 1. As illustrated in  FIG.  6   , the involute angle (involute-start-point angle) at the point of connection (an involute start point  303 ) between the small arc part  301  in the first tier and the outer-surface involute curve is denoted by φos(1). Furthermore, the involute angle (involute-start-point angle) at the point of connection (an involute start point  303   a ) between the small arc part  301   a  in the second tier and the outer-surface involute curve is denoted by φos(2). Furthermore, the involute angle (involute-start-point angle) at the point of connection (an involute start point  303   b ) between the small arc part  301   b  in the third tier and the outer-surface involute curve is denoted by φos(3). Here, the involute-start-point angles in the respective tiers are expressed as φos(1)&gt;φos(2)&gt;φos(3). Since the number of tiers is n (where n≥2), the above expression regarding the involute-start-point angles in the respective tiers is generalized as φos(1)&gt;φos(2) . . . &gt;φos(n). 
     The above configuration of the fixed scroll  30  regarding the involute-start-point angles of the outer-surface involute curves also applies to the scroll center portion of the orbiting scroll  40 , which is not illustrated. Specifically, letting the involute-start-point angle of the outer-surface involute curve in the first tier be φos(1); the involute-start-point angle of the outer-surface involute curve in the second tier be φos(2); and the involute-start-point angle of the outer-surface involute curve in the third tier be φos(3), a relationship of φos(1)&gt;φos(2)&gt;φos(3) is established. 
       FIG.  7    includes enlarged plan views of the scroll start portions of the fixed scroll and the orbiting scroll included in the scroll compressor according to Embodiment 1. Referring to  FIG.  7   , the shapes of the fixed scroll  30  and the orbiting scroll  40  of the scroll compressor  1  will now be described in detail. Diagram (a) of  FIG.  7    illustrates a state (at a crank angle ψ of 0) established when the second compression chambers  5   a   2  are just opened to the innermost compression chamber  5   a   1  located at the center. Diagram (b) of  FIG.  7    illustrates a state (at a crank angle ψ of 0+15 degrees) established when the orbiting scroll undergoes the orbital motion by 15 degrees after the opening. Diagram (c) of  FIG.  7    illustrates a state (at a crank angle ψ of 0+30 degrees) established when the orbiting scroll undergoes the orbital motion by 30 degrees after the opening. Diagram (d) of  FIG.  7    illustrates a state (at a crank angle of 0+45 degrees) established when the orbiting scroll undergoes the orbital motion by 45 degrees after the opening. Diagram (e) of  FIG.  7    illustrates a state (at a crank angle ψ of 0+60 degrees) established when the orbiting scroll undergoes the orbital motion by 60 degrees after the opening. Diagram (f) of  FIG.  7    illustrates a state (at a crank angle ψ of 0+90 degrees) established when the orbiting scroll undergoes the orbital motion by 90 degrees after the opening. 
     As illustrated in  FIG.  7   , as the crank angle increases, the contact points, t, between the scroll start portion of the fixed scroll  30  and the scroll start portion of the orbiting scroll  40  each move in the direction in which the involute angles of the involute curves forming the lateral face of the scroll wrap decrease. At the crank angle ψ of 0 in the just-opened state illustrated in diagram (a) of  FIG.  7   , each contact point t is at the involute-start-point angle φos(1) of the involute curve in the first tier. Specifically, when the contact point t reaches the involute-start-point angle φos(1) of the involute curve in the first tier, the fixed scroll wrap  30   b  and the orbital scroll wrap  40   b  go out of contact with each other in the first tier, whereby the second compression chamber  5   a   2  is made to communicate with the innermost compression chamber  5   a   1  through a passage  50  (see  FIG.  9    to be referred to below). The passage  50  is provided as a gap between the first tier and the second tier. Consequently, the high-pressure refrigerant in the innermost compression chamber  5   a   1  flows into the second compression chambers  5   a   2 , whereby the pressure starts to be equalized therebetween. 
     The above situation where “the fixed scroll wrap  30   b  and the orbital scroll wrap  40   b  go out of contact with each other in the first tier, whereby the two compression chambers  5   a , namely the second compression chamber  5   a   2  and the innermost compression chamber  5   a   1  that are not made to communicate with each other before the scroll wraps  30   b  and  40   b  go out of contact with each other, are made to communicate with each other through the passage  50  (see  FIG.  9    to be referred to below) provided as the gap between the first tier and the second tier” is hereinafter simply expressed as “the first tier is opened”. Such an expression also applies to the cases of the other tiers. 
     As the crank angle increases from that in diagram (a) of  FIG.  7   , the contact point t further moves in the direction in which the involute angles of the involute curves decrease. When the contact point t reaches the involute-start-point angle φos(2) of the involute curve, a passage  51  is further provided as the gap between the second tier and the third tier and makes the second compression chamber  5   a   2  opened to the innermost compression chamber  5   a   1 . Thus, the second compression chamber  5   a   2  is made to communicate with the innermost compression chamber  5   a   1  in a graded manner: first in the first tier, subsequently in the second tier, and eventually in the third tier. 
     When the first tier is opened and the pressure starts to be equalized between the innermost compression chamber  5   a   1  and the second compression chamber  5   a   2 , the second tier and the third tier are still kept in contact with the lateral face of the counterpart scroll wrap. As the crank angle further increases and by the time when the third tier is opened, the pressure equalization between the innermost compression chamber  5   a   1  and the second compression chamber  5   a   2  is complete. In such a state, the scroll start portion is out of contact with the counterpart scroll wrap over the entirety from the first tier to the third tier. 
     Now, a load that acts on the scroll start portion will be described. A known configuration employing a non-tiered scroll start portion and a variable crank mechanism will first be described as a comparative example. The scroll start portion in the known configuration has a shape obtained by omitting the first tier and the second tier from the scroll start portion according to Embodiment 1, so that the third tier extends continuously from the base to the tip. The following description relates to a case of a pressure acting on the scroll start portion of the orbital scroll wrap but also applies to a case of a pressure acting on the scroll start portion of the fixed scroll wrap. 
       FIG.  8    illustrates how the pressure acts on the scroll start portion according to the comparative example at the start of pressure equalization. In the known configuration as the comparative example, when the second compression chamber  5   a   2  is made to communicate with the innermost compression chamber  5   a   1 , a load generated by the difference between the pressure in the second compression chamber  5   a   2  and the pressure in the innermost compression chamber  5   a   1  acts on the orbital scroll wrap  40   b . When the two chambers are made to communicate with each other, the orbital scroll wrap  40   b  and the fixed scroll wrap  30   b  are out of contact with each other. Therefore, as illustrated in  FIG.  8   , the orbital scroll wrap  40   b  is tilted by receiving the load generated by the above pressure difference, and a great stress is generated at the base of the orbital scroll wrap  40   b . Such a stress is also generated in a compressor including no variable crank mechanism and that operates with the lateral faces of the respective scroll wraps being not kept in contact with each other. 
     In contrast, according to Embodiment 1, since the scroll start portion has a tiered bulbous shape, the stress generated at the base of the scroll wrap is reduced. Such a mechanism will now be described with reference to  FIGS.  9 ,  10 , and  11   . 
       FIG.  9    illustrates how the pressure acts on the scroll start portion of the scroll compressor according to Embodiment 1 at the start of pressure equalization.  FIG.  10    illustrates how the pressure acts on the scroll start portion of the scroll compressor according to Embodiment 1 after the completion of pressure equalization. 
     Since the scroll compressor  1  according to Embodiment 1 includes a variable crank mechanism, the lateral face of the orbital scroll wrap  40   b  and the lateral face of the fixed scroll wrap  30   b  are in contact with each other when the scroll compressor  1  is in operation. However, when the contact point t reaches the involute-start-point angle φos(1) of the involute curve in the first tier, as described above, the fixed scroll wrap  30   b  and the orbital scroll wrap  40   b  go out of contact with each other in the first tier, whereby the pressure starts to be equalized through the passage  50 . Immediately after the start of such pressure equalization, as illustrated in  FIG.  9   , the orbital scroll wrap  40   b  is in contact with and is thus supported by the lateral face of the fixed scroll wrap  30   b . Therefore, the fixed scroll wrap  30   b  exerts a reaction force R against the load, P, generated by the pressure difference between the innermost compression chamber  5   a   1  and the second compression chamber  5   a   2  and acting on the orbital scroll wrap  40   b . Such a reaction force R reduces the stress generated at the base of the orbital scroll wrap  40   b.    
     When the orbital scroll wrap  40   b  is out of contact with the fixed scroll wrap  30   b  over the entirety from the first tier to the third tier, as illustrated in  FIG.  10   , the pressure is equal between the second compression chamber  5   a   2  and the innermost compression chamber  5   a   1 , that is, the pressure difference between the innermost compression chamber  5   a   1  and the second compression chamber  5   a   2  is zero. Therefore, no stress is generated at the base of the orbital scroll wrap  40   b.    
       FIG.  11    illustrates the change in the stress generated at the base of the scroll start portion versus the change in the crank angle in the scroll compressor according to Embodiment 1. In  FIG.  11   , the horizontal axis represents crank angle, and the vertical axis represents stress. The solid line represents the stress generated at the scroll start portion according to Embodiment 1. The broken line represents the stress generated at the scroll start portion in the known configuration. 
     In Embodiment 1, as the crank angle increases as represented by the solid line in  FIG.  11   , the stress generated at the scroll start portion increases. When the crank angle reaches the involute start point angle in the first tier (in this case, ψ of 0-0.3π), the first tier is opened, whereby pressure equalization starts. Accordingly, the stress generated at the base of the scroll start portion starts to be reduced. In the known configuration, the stress keeps increasing as represented by the broken line. When the involute start point angle (in this case, ψ is 0) is reached, an increase in the stress occurs because the scroll start portion loses its support. Subsequently, pressure equalization starts, whereby the stress starts to be reduced. 
     As can be seen from  FIG.  11   , the maximum stress generated at the base of the scroll start portion in the known configuration is σ2, whereas the maximum stress generated in Embodiment 1 is σ1, which is smaller than in the known configuration. 
     Now, a structure that further reduces the stress generated at the base of the scroll start portion will be described. In Embodiment 1, the involute-start-point angles in the respective tiers that define the tiered bulbous shape of the scroll start portion are in a relationship of φos(1)&gt;φos(2) . . . &gt;φos(n), as described above. Furthermore, the tiered bulbous shape of the scroll start portion according to Embodiment 1 satisfies a relationship of 0.3π&lt;φos(1)-φos(n)&lt;0.7π. Satisfying this relationship further reduces the stress generated at the base of the scroll start portion. Before describing such a mechanism, how the tiered bulbous shape of the scroll start portion varies with the value of “φos(1)-φos(n)” will first be described. 
       FIG.  12    is an enlargement of the scroll start portion of the scroll compressor according to Embodiment 1 in a case where φos(1)-φos(n) is 0.2π.  FIG.  13    is an enlargement of the scroll start portion of the scroll compressor according to Embodiment 1 in a case where φos(1)-φos(n) is 0.5π. As can be seen from the comparison between  FIGS.  12  and  13   , with the increase in the value of φos(1)-φos(n), the position of the small arc part  301  in the first tier is shifted toward the end of the scroll. 
     Now, why satisfying the above relationship further reduces the stress generated at the base of the scroll start portion will be described. First, a case of the known configuration as the comparative example employing a non-tiered scroll start portion will be described. 
       FIG.  14    includes diagrams illustrating the relationship between the direction in which a load acts on the scroll start portion in the known configuration taken as the comparative example and the thickness of the scroll start portion that receives the load. The arrows illustrated in  FIG.  14    each represent the direction of the load, which is determined by integrating the differential pressure acting on the bulbous part. The length of each arrow represents the thickness of the scroll start portion in a section of the scroll start portion that is taken along the arrow, in other words, the thickness of the scroll start portion in the direction of the load. What the arrow represents is the same for the diagrams in  FIG.  15    to be referred to below. In  FIG.  14   , only the first tier and the n-th tier are illustrated, with the other tiers not illustrated, which also applies to  FIGS.  15  to  17    to be referred to below. 
     The stress at the base of the scroll start portion increases in proportion to the working load divided by the section modulus. Therefore, making the section modulus in the direction of the load satisfactorily large is highly effective in terms of strength improvement for the scroll start portion. To make the section modulus in the direction of the load satisfactorily large, the thickness of the scroll start portion in the direction of the load is to be made satisfactorily large. However, in the known configuration illustrated in  FIG.  14   , as the crank angle ψ increases from 0-0.5π to 0-0.2π, the thickness of the scroll start portion in the direction of the load is reduced. At a crank angle ψ of 0 where the second compression chamber  5   a   2  is made to communicate with the innermost compression chamber  5   a   1 , the thickness of the scroll start portion is further reduced. In other words, the second compression chamber  5   a   2  is made to communicate with the innermost compression chamber  5   a   1  at a small section modulus. Therefore, a great stress is generated at the base of the scroll start portion. 
       FIG.  15    includes diagrams illustrating the relationship between the direction in which a load acts on the scroll start portion of the scroll compressor according to Embodiment 1 and the thickness of the scroll start portion that receives the load in the case where φos(1)-φos(n) is 0.2π.  FIG.  16    includes diagrams illustrating the relationship between the direction in which a load acts on the scroll start portion of the scroll compressor according to Embodiment 1 and the thickness of the scroll start portion that receives the load in the case where φos(1)-φos(n) is 0.5π.  FIG.  17    includes diagrams illustrating the relationship between the direction in which a load acts on the scroll start portion of the scroll compressor according to Embodiment 1 and the thickness of the scroll start portion that receives the load in the case where φos(1)-φos(n) is 0.7π. 
     In the case illustrated in  FIG.  15    where φos(1)-φos(n) is 0.2π, the first tier is opened to start pressure equalization at a crank angle ψ of 0-0.2π, earlier than in the known configuration illustrated in  FIG.  14    by 0.2π. That is, by the time when the crank angle ψ reaches 0, pressure equalization is complete, generating no load. The thickness of the scroll start portion in the direction of the load acting at the opening of the first tier is greater than the thickness at the crank angle ψ of 0 in the case illustrated in  FIG.  14   . Therefore, the stress generated at the scroll start portion immediately before the opening is smaller than in the known configuration. However, between the case illustrated in  FIG.  15    and the known configuration illustrated in  FIG.  14   , since the difference in the crank angle established at the time of opening is 0.2π, the difference in the thickness of the scroll start portion in the direction of the load acting immediately before the opening is small. Correspondingly, the effect of strength improvement to be achieved is small. 
     In the case illustrated in  FIG.  16    where φos(1)-φos(n) is 0.5π, the first tier is opened to start pressure equalization at a crank angle ψ of 0-0.5π, earlier than in the case illustrated in  FIG.  15    by 0.3π. Therefore, the stress generated at the scroll start portion is smaller than in the case illustrated in  FIG.  15   . Since the thickness of the scroll start portion in the direction of the load acting immediately before the opening is significantly greater than in the known configuration illustrated in  FIG.  14   , a high degree of strength improvement is achieved. 
     If the case where φos(1)-φos(n) is 0.2π is plotted in  FIG.  11   , the stress is illustrated to start decreasing at a crank angle of −0.2π. If the case where φos(1)-φos(n) is 0.5π is plotted in  FIG.  11   , the stress is illustrated to start decreasing at a crank angle of −0.5π. Thus, the maximum stress that acts on the scroll start is variable with the value of φos(1)-φos(n). 
     In the case illustrated in  FIG.  17    where φos(1)-φos(n) is 0.7π, the first tier is opened to start pressure equalization at a crank angle ψ of 0-0.7π, earlier than in the case illustrated in  FIG.  16    by 0.2π. Therefore, the stress generated at the scroll start portion is smaller than in the case illustrated in  FIG.  16   . However, the increment, obtained by changing φos(1)-φos(n) from 0.5π to 0.7π, in the thickness of the scroll start portion in the direction of the load acting immediately before the opening is smaller than the increment obtained by changing φos(1)-φos(n) from 0.2π to 0.5π. Therefore, the degree of strength improvement achieved by changing φos(1)-φos(n) from 0.5π to 0.7π is lower than by changing φos(1)-φos(n) from 0.2π to 0.5π. Such a situation means as follows. If the difference in the involute-start-point angle between the first tier and the n-th tier (φos(1)-φos(n)) is small, the increment in the section modulus is very small, resulting in a degree of strength improvement that does not meet the cost increase required for the fabrication of the tiered structure. Furthermore, as φos(1)-φos(n) is made greater, the degree of strength improvement becomes lower, that is, the strength does not infinitely increase. Moreover, as φos(1)-φos(n) is made greater, the performance at a high compression ratio becomes lower. Under such circumstances, if φos(1)-φos(n)&lt;0.7π is satisfied, a high degree of strength improvement is achieved with the minimum performance deterioration. 
       FIG.  18    illustrates the result of a strength analysis conducted on the scroll start portion of the scroll compressor according to Embodiment 1. In  FIG.  18   , the horizontal axis represents φos(1)-φos(n), and the vertical axis represents the ratio of stress reduction at the scroll start portion. As can be seen from  FIG.  18   , the stress is reduced significantly in a range of φos(1)-φos(n) from 0.3π to 0.7π, whereas the effect of stress reduction is substantially saturated in a range over 0.7π. 
     That is, if the bulbous shape of the scroll start portion is designed to satisfy the relationship of 0.3π&lt;φos(1)-φos(n)&lt;0.7π where the degree of reduction in the ratio of stress reduction is high, a satisfactory degree of strength improvement is achieved. 
     The greater the difference in the involute-start-point angle between the first tier and the n-th tier (φos(1)-φos(n)) is made, the more assuredly the pressure equalization between the innermost compression chamber  5   a   1  and the second compression chamber  5   a   2  is complete before the n-th tier is opened. The limit for completing the pressure equalization between the innermost compression chamber  5   a   1  and the second compression chamber  5   a   2  before the n-th tier is opened is expressed by the relationship of φos(1)-φos(n)&gt;0.3π. To put it the other way round, if φos(1)-φos(n) 0.3π, the n-th tier is opened before the completion of pressure equalization between the innermost compression chamber  5   a   1  and the second compression chamber  5   a   2 . The n-th tier that is opened before the completion of pressure equalization between the innermost compression chamber  5   a   1  and the second compression chamber  5   a   2  is no longer supported by the counterpart scroll wrap. Therefore, the relationship of φos(1)-φos(n)&gt;0.3π is to be satisfied. Thus, the n-th tier is kept supported by the counterpart scroll wrap until the pressure equalization between the innermost compression chamber  5   a   1  and the second compression chamber  5   a   2  is complete to make the pressure difference between the two 0. 
     The sizes of the small circles in the respective tiers of the scroll start portion are determined with no restrictions. However, as illustrated in  FIG.  3   , the scroll start portion  40   e  of the orbital scroll wrap  40   b  overlaps the discharge port  30   f  and therefore closes a part of the passage provided by the discharge port  30   f . To avoid such a situation, the small circle in the first tier of the scroll start portion  40   e  of the orbital scroll wrap  40   b  may be set to such a small value as not to close the discharge port  30   f . If the thickness of the first tier of the scroll start portion  40   e  is reduced, the discharge port  30   f  is not closed. Consequently, the discharge pressure loss is reduced, which produces a secondary effect of performance improvement. 
     Note that reducing the size of the small circle in the first tier leads to a reduction in the strength of the first tier. Such a reduction in the strength may be solved by employing a configuration illustrated in  FIG.  19   . 
       FIG.  19    is a schematic enlargement of a longitudinal section of the scroll start portion of the scroll compressor according to Embodiment 1 and peripheries thereof. 
     As illustrated in  FIG.  19   , the curvature radius, R 1 , at the base of the first tier is greater than the curvature radius, Rn, at the base of the n-th tier. Thus, the concentration of stress at the base of the first tier is eased, and a satisfactory strength is provided. The reason the curvature radius R 1  at the base of the first tier can be made greater than the curvature radius at the base of the n-th tier is as follows. In the compression chamber  5   a , a passage that allows leakage tends to be formed at the base of the n-th tier. Therefore, if the curvature radius at the base of the n-th tier is made large, refrigerant leakage occurs, resulting in performance deterioration. On the other hand, such a passage that allows leakage is not formed at the base of the first tier, not leading to performance deterioration. Therefore, the curvature radius R 1  in the first tier can be made greater than the curvature radius Rn at the base of the n-th tier. The curvature radius in each of the second to (n−1)-th tiers can also be made greater than the curvature radius Rn at the base of the n-th tier for the same reason. 
     The ratio of the total height, Hn−1, of the first to (n−1)-th tiers of the scroll start portion to the total height, Hn, of the first to n-th tiers may be set to 25% to 50%. If the ratio is below 25%, the area of the passage for pressure equalization between the innermost compression chamber  5   a   1  and the second compression chamber  5   a   2  is insufficient. In such a case, when the n-th tier is opened, there remains a pressure difference between the innermost compression chamber  5   a   1  and the second compression chamber  5   a   2 , failing in achieving a satisfactory degree of strength improvement. If the ratio is above 50%, the stress generated at the base of each of the first to (n−1)-th tiers increases. In such a case, the first to (n−1)-th tiers may be damaged before the base of the n-th tier is damaged. 
     In the above description, each of the fixed scroll  30  and the orbiting scroll  40  includes the tiered scroll start portion. Alternatively, only one of the fixed scroll  30  and the orbiting scroll  40  may include the tiered scroll start portion. 
     According to Embodiment 1, the scroll compressor is configured to compress a working medium in the plurality of compression chambers  5   a  defined between the orbital scroll wrap  40   b  of the orbiting scroll  40  and the fixed scroll wrap  30   b  of the fixed scroll  30  that are made to mesh with each other. The working medium is compressed when the orbiting scroll  40  driven through the main shaft  7  undergoes an orbital motion about the fixed scroll  30 . The orbital scroll wrap  40   b  and the fixed scroll wrap  30   b  include respective scroll start portions each having a bulbous shape defined by connecting the involute start point of the outer-surface involute curve and the involute start point of the inner-surface involute curve to each other with a plurality of arcs. At least one of the scroll start portions has a tiered shape in which an n number (where n≥2) of tiers each having the bulbous shape are stacked in the axial direction of the main shaft  7 . Letting the involute-start-point angles of the outer-surface involute curves in the respective tiers of the scroll start portion having the tiered shape be φos(1), φos(2), φos(3), . . . , and φos(n) in order from the tip toward the base of the scroll start portion, the following relationships are satisfied: φos(1)&gt;φos(2)&gt;φos(3)&gt; . . . &gt;φos(n); and 0.3π&lt;φos(1)-φos(n)&lt;0.7π. 
     Since the scroll start portion has the above tiered shape, among the plurality of compression chambers  5   a , the innermost compression chamber  5   a   1  and the second compression chamber  5   a   2  on the radially outer side of the innermost compression chamber  5   a   1  are made to communicate with each other in a graded manner. Thus, the stress generated at the base of the scroll start portion is reduced. Furthermore, satisfying the relationship of 0.3π&lt;φos(1)-φos(n)&lt;0.7π brings a satisfactory degree of strength improvement for the scroll start portion that meets the cost increase required for the fabrication of the tiered structure. Furthermore, the n-th tier is kept supported by the counterpart scroll wrap until the pressure equalization between the innermost compression chamber  5   a   1  and the second compression chamber  5   a   2  is complete to make the pressure difference between the two 0. 
     Embodiment 2 
     Embodiment 2 will now be described, except some of the features that are the same as those described in Embodiment 1. 
     When the scroll compressor  1  starts to operate, the orbiting scroll  40  and the fixed scroll  30  each come to have a high temperature, specifically 100 degrees C. or higher. Accordingly, the orbital scroll wrap  40   b  and the fixed scroll wrap  30   b  undergo thermal expansion. If the orbiting scroll  40  and the fixed scroll  30  are made of respective materials having different coefficients of linear expansion: for example, one of the two is aluminum while the other is cast iron, a high pressure may be applied to the base of each of the scroll start portions as to be described in detail below. 
     Embodiment 2 relates to a technique of reducing the stress generated at the base of each of the scroll start portions because of the difference in the coefficient of linear expansion between the material forming the orbiting scroll  40  and the material forming the fixed scroll  30 . In Embodiment 2, the coefficient of linear expansion of the orbiting scroll  40  is greater than the coefficient of linear expansion of the fixed scroll  30 . 
       FIG.  20    schematically illustrates a lateral section of a compression unit of the scroll compressor according to Embodiment 2.  FIG.  21    is a perspective view of one of reliefs provided in the scroll compressor according to Embodiment 2. 
     The fixed scroll wrap  30   b  has an inner-surface involute  30   c  (hereinafter referred to as fixed-inner-surface involute  30   c ). The fixed-inner-surface involute  30   c  has the reliefs,  30   c   1 . As illustrated in  FIG.  21   , the reliefs  30   c   1  are depressions provided in the fixed-inner-surface involute  30   c  and extend parallel to the axial direction. The reliefs  30   c   1  are provided to make the fixed-inner-surface involute  30   c  and an outer-surface involute  40   d , which is of the orbital scroll wrap  40   b , be partially out of contact with each other (the outer-surface involute  40   d  is hereinafter referred to as orbital-outer-surface involute  40   d ). 
     The regions where the reliefs  30   c   1  are to be provided are defined by the following seven parameters: 
     the involute angle at the start point,  30   c   1   a , of each relief  30   c   1 : φia3; 
     the involute angle at the end point,  30   c   1   b , of each relief  30   c   1 : φib3; 
     the involute angle at the involute end point,  30   c   2 , of the fixed-inner-surface involute  30   c : φie3; 
     the involute-start-point angle of the inner-surface involute in the n-th tier of the fixed scroll wrap  30   b : φis3(n); 
     the involute-start-point angle of the outer-surface involute in the first tier of the orbital scroll wrap  40   b : φos4(1); 
     the involute-start-point angle of the outer-surface involute in the n-th tier of the orbital scroll wrap  40   b : φos4(n); and 
     the number of contact points between the orbital-outer-surface involute  40   d  and the fixed-inner-surface involute  30   c  (the contact points are hereinafter referred to as orbital-outer-surface contact points): m=[(φie3−φis3(n))/2π], where [ ] indicates that the decimals are to be rounded down. 
     Letting the serial numbers of the orbital-outer-surface contact points be i (1, 2, 3, . . . , and m from the inner side), where m≥2, the reliefs  30   c   1  are provided in regions where the following relationships are satisfied: 
     “i≥2”, “φia3&lt;(φos1(n)+π)+2π×(i−1)”, and “φib3&gt;(φos1(1)+π)+2π×(i−1)”.  FIG.  20    illustrates a case of m=3. The following description relates to the case of m=3. 
     Specifically, the reliefs  30   c   1  are provided such that when the crank angle is between the involute-start-point angle φos(1) of the orbital-outer-surface involute  40   d  in the first tier and the involute-start-point angle φos(n) of the orbital-outer-surface involute  40   d  in the n-th tier, the orbital-outer-surface involute  40   d  and the fixed-inner-surface involute  30   c  are out of contact with each other at the orbital-outer-surface contact points expressed as i=2 or greater. In other words, the reliefs  30   c   1  are provided in the fixed-inner-surface involute  30   c  such that while the orbiting scroll  40  is undergoing the orbital motion from the crank angle at which the first tier is opened to the crank angle at which the n-th tier is opened, the orbital-outer-surface contact points, excluding the innermost one, on the orbital scroll wrap  40   b  that is made of the material having the greater coefficient of linear expansion are out of contact. 
     An effect produced by the above configuration will now be described. First, a configuration with no relief  30   c   1  will be described as a comparative example. The following description relates to the case of m=3. 
       FIG.  22    schematically illustrates a lateral section of a compression unit according to the comparative example, with an orbiting scroll being made to uneccentrically mesh with a fixed scroll, and also illustrates gaps produced between an orbital-outer-surface involute and a fixed-inner-surface involute at room temperature.  FIG.  23    schematically illustrates a lateral section of the compression unit according to the comparative example, with the orbiting scroll being made to uneccentrically mesh with the fixed scroll, and also illustrates gaps produced between the orbital-outer-surface involute and the fixed-inner-surface involute during operation.  FIG.  24    illustrates the gap sizes, δ0, of the respective gaps produced at room temperature in the compression unit according to the comparative example, with the orbiting scroll being made to eccentrically mesh with the fixed scroll.  FIG.  25    illustrates the changes, δa, in the sizes of the respective gaps produced in the compression unit according to the comparative example, between the gap sizes at room temperature and the gap sizes during operation.  FIG.  26    illustrates the gap sizes, δs, of the respective gaps produced during operation in the compression unit according to the comparative example. In  FIGS.  24  to  26   , the horizontal axis represents the position of the gaps, and the vertical axis represents the gap size (μm). 
     In  FIG.  22   , the points denoted by i are the orbital-outer-surface contact points. The orbital-outer-surface contact points refer to the points where the orbital-outer-surface involute  40   d  comes into contact with the fixed-inner-surface involute  30   c  when the orbiting scroll  40  is made eccentric to the fixed scroll  30 . The orbital-outer-surface contact points are serially numbered in the direction from the scroll start portion toward the radially outer side. In the illustrated case, there are three points: i=1, i=2, and i=3. The points denoted by i2 are referred to as orbital-inner-surface contact points. The orbital-inner-surface contact points refer to the points where the orbital-inner-surface involute  40   c  comes into contact with the fixed-outer-surface involute  30   d  when the orbiting scroll  40  is made eccentric to the fixed scroll  30 . The orbital-inner-surface contact points are serially numbered in the direction from the scroll start portion toward the radially outer side. In the illustrated case, there are three points: i2=1, i2=2, and i2=3. Hereinafter, the contact point numbered as i=1 is expressed as contact point (i=1), which also applies to the other numbered contact points. 
     As illustrated in  FIG.  22   , the number of gaps produced between the lateral faces of the orbital scroll wrap  40   b  and the fixed scroll wrap  30   b  when the orbiting scroll  40  is made to uneccentrically mesh with the fixed scroll  30  totals 2×3. Specifically, three gaps are produced between the orbital-outer-surface involute  40   d  and the fixed-inner-surface involute  30   c , and three gaps are produced between the orbital-inner-surface involute  40   c  and the fixed-outer-surface involute  30   d . The gaps at the 2×3 positions are of the same size at room temperature as illustrated in  FIG.  22   . Referring to  FIG.  22   , the gaps between the orbital-outer-surface involute  40   d  and the fixed-inner-surface involute  30   c  are denoted by δo1, δo2, and δo3 in the direction from the scroll start portion toward the radially outer side. Furthermore, the gaps between the orbital-inner-surface involute  40   c  and the fixed-outer-surface involute  30   d  are denoted by δi1, δi2, and δi3 in the direction from the scroll start portion toward the radially outer side. 
     When the orbiting scroll  40  is made eccentric in the direction of the arrow illustrated in  FIG.  22   , the orbiting scroll  40  and the fixed scroll  30  come into contact with each other with no gaps at the orbital-outer-surface contact points (i=1, 2, and 3) and at the orbital-inner-surface contact points (i2=1, 2, and 3). That is, when the orbiting scroll  40  is made eccentric in the direction of the arrow at room temperature, as illustrated in  FIG.  24   , the gap size δ0 is 0 for all the gaps δo1, δo2, δo3, δi1, δi2, and δi3. 
     However, when the scroll compressor  1  starts to operate, as described above, the orbiting scroll  40  and the fixed scroll  30  each come to have a high temperature, specifically 100 degrees C. or higher. Accordingly, the orbital scroll wrap  40   b  and the fixed scroll wrap  30   b  undergo thermal expansion. The orbital scroll wrap  40   b , which is made of a material having a greater coefficient of linear expansion than the fixed scroll wrap  30   b , expands to a greater extent than the fixed scroll wrap  30   b  as illustrated in  FIG.  23   . 
     The changes δa in the gap sizes during operation from the gap sizes at room temperature increase in the direction from the scroll start portion toward the radially outer side as illustrated in  FIG.  23   . Specifically, in the direction of eccentricity of the orbiting scroll  40  from the scroll start portion that is represented by the arrow in  FIG.  23   , the sizes of the gaps δo1, δo2, and δo3 are smaller during operation than at room temperature, and the degrees of reduction in the gap sizes increase toward the radially outer side. Therefore, for the gaps δo1, δo2, and δo3, the changes δa obtained by subtracting the gap sizes at room temperature from the gap sizes during operation are negative values as illustrated in  FIG.  25    and increase toward the radially outer side. 
     In the other direction from the scroll start portion that is opposite to the direction of eccentricity of the orbiting scroll  40 , the sizes of the gaps δi1, δi2, and δi3 are greater during operation, illustrated in  FIG.  23   , than at room temperature, and the degrees of increase in the gap sizes increase toward the radially outer side. Therefore, for the gaps δi1, δi2, and δi3, the changes δa obtained by subtracting the gap sizes at room temperature from the gap sizes during operation are positive values as illustrated in  FIG.  25    and increase toward the radially outer side. 
     The relationship of the sizes of the gaps during operation is expressed as δo3&lt;δo2&lt;δo1&lt;δi1&lt;δi2&lt;δi3. What have been described above are the gaps at room temperature and the gaps during operation with the orbiting scroll  40  being made to uneccentrically mesh with the fixed scroll  30 . In the actual operation, however, the orbiting scroll  40  is made eccentric in the direction of the arrow illustrated in  FIG.  23   . 
     When the orbiting scroll  40  in the state illustrated in  FIG.  23    is made eccentric in the direction of the arrow, the orbital-outer-surface contact point (i=3) at the smallest gap δo3 in  FIG.  23    first comes into contact with the fixed-inner-surface involute  30   c . Meanwhile, the other contact points (i=1 and 2) are out of contact with the fixed-inner-surface involute  30   c , leaving gaps therebetween. At the contact points (i2=1, 2, and 3) on the side opposite the side toward which the orbiting scroll  40  is made eccentric, the orbital-inner-surface involute  40   c  is out of contact with the fixed-outer-surface involute  30   d , leaving gaps therebetween. To summarize the gap sizes δs of the gaps produced with the orbiting scroll  40  illustrated in  FIG.  23    being made eccentric in the direction of the arrow and being expanded during operation, referring to  FIG.  26   , the gap size δs of the gap δo3 is 0, whereas the gap sizes δs of the other gaps δo2, δo1, δi1, δi2, and δi3 are not 0 and increase in that order. 
     The gap δo1 and the gap δi1 are the gaps at the scroll start portion. If the scroll compressor is operated with the gap sizes δs of the gap δo1 and the gap δi1 being not 0, the following problem arises. Before the pressure is equalized between the innermost compression chamber  5   a   1  and the second compression chamber  5   a   2 , the fixed scroll wrap  30   b  and the orbital scroll wrap  40   b  lose their respective supports at the respective scroll start portions. Therefore, great stresses are generated at the bases of the respective scroll start portions. Hence, the gap sizes δs of the gap δo1 and the gap δi1 at the scroll start portion are required to be 0 or small. To make the gap sizes δs of the gap δo1 and the gap δi1 small, it is effective to make the gap size δ0 at room temperature as large as possible for each of the gap δo3 and the gap δo2, that is, for all but the gap δo1 at the scroll start portion among the gaps δo3, δo2, and δo1 on the side toward which the orbiting scroll  40  is made eccentric. 
     Accordingly, in Embodiment 2, the inner-surface involute  30   c  is designed in view of the expansion of the orbital scroll wrap  40   b  that occurs during operation. Thus, the gap δo3 and the gap δo2 produced at room temperature and when the orbiting scroll  40  is made to eccentrically mesh with the fixed scroll  30  are originally made satisfactorily large, as illustrated in  FIG.  27   . 
       FIG.  27    illustrates the gap sizes δ0 of the respective gaps produced at room temperature in the compression unit of the scroll compressor according to Embodiment 2, with the orbiting scroll being made to eccentrically mesh with the fixed scroll.  FIG.  28    illustrates the effect produced by the reliefs provided in the compression unit of the scroll compressor according to Embodiment 2, that is, the differences, δb, between the gap sizes at room temperature in the comparative example and the gap sizes during operation in Embodiment 2. The values in  FIG.  28    are the sums of the values in  FIG.  25    and the values in  FIG.  27   .  FIG.  29    illustrates the gap sizes δs of the respective gaps produced during operation in the compression unit of the scroll compressor according to Embodiment 2. In  FIGS.  27  to  29   , the horizontal axis represents the position of the gaps, and the vertical axis represents the gap size (μm). 
     In Embodiment 2, the fixed-inner-surface involute  30   c  has the reliefs  30   c   1 . Therefore, as illustrated in  FIG.  27   , the gap δo2 and the gap δo3 at room temperature each originally have a satisfactorily large gap size δ0. Referring to  FIG.  28   , the difference δb in the gap size during operation from the gap size at room temperature in the comparative example is negative for the gap δo1 at the innermost contact point (i=1) on the orbital-outer-surface involute, indicating that the gap size during operation is smaller than the gap size at room temperature in the comparative example. The differences δb for the gap δo2 and the gap δo3 at the other contact points (i=2 and 3) on the orbital-outer-surface involute are positive, indicating that the gap sizes are greater. Hence, when the orbiting scroll  40  is made eccentric to the fixed scroll  30  and is expanded, the orbital-outer-surface contact point (i=1) at the smallest gap δo1 first comes into contact with the fixed-inner-surface involute  30   c , whereby the gap size δs of the gap δo1 becomes 0 as illustrated in  FIG.  29   . Meanwhile, the other contact points (i=2 and 3) on the orbital-outer-surface involute are out of contact with the fixed-inner-surface involute  30   c , leaving gaps therebetween. 
     The above state where the gap size δs of the gap δo1 is 0 is maintained at least from when the first tier of the scroll start portion is opened until the n-th tier is opened. That is, before the pressure equalization between the innermost compression chamber  5   a   1  and the second compression chamber  5   a   2  is complete, the scroll start portion  40   e  of the orbital scroll wrap  40   b  is supported by the lateral face of the fixed scroll wrap  30   b . Therefore, the generation of a great stress at the base of the scroll start portion is suppressed. Thus, strength improvement is achieved. Meanwhile, the gap size δs of the gap δi1 produced at the innermost contact point (i2=1) on the orbital-inner-surface involute  40   c , in other words, the gap size δs of the gap δi1 at the scroll start portion  30   e  of the fixed scroll wrap  30   b , is not reduced to 0 as illustrated in  FIG.  29   , leaving a gap. If there remains such a gap, the scroll start portion  30   e  of the fixed scroll wrap  30   b  is deformed in such a manner as to be tilted when receiving a load generated by the difference between the pressure in the second compression chamber  5   a   2  and the pressure in the innermost compression chamber  5   a   1 . Nevertheless, the scroll start portion  30   e  of the fixed scroll wrap  30   b  that is deformed in such a manner as to be tilted is supported by the lateral face of the orbital scroll wrap  40   b  when the scroll start portion  30   e  is deformed by the amount equivalent to the above gap size δs. In such a configuration, the smaller the gap size δs, the smaller the stress generated at the base of the scroll start portion. Comparing the gap size δs of the gap δi1 illustrated in  FIG.  29    and the gap size δs of the gap δi1 in the case illustrated in  FIG.  26    where no relief  30   c   1  is provided, the gap size δs of the gap δi1 in  FIG.  29    is smaller. That is, providing the reliefs  30   c   1  produces an effect of strength improvement for the scroll start portion  30   e  of the fixed scroll wrap  30   b  as well. 
     In Embodiment 2, the relief  30   c   1  is provided at each of all the contact points on the orbital-outer-surface involute but the innermost one. Alternatively, the relief  30   c   1  may be provided only at the outermost contact point (i=3). Specifically, the relief  30   c   1  only needs to be provided in the fixed-inner-surface involute  30   c  such that while the orbiting scroll  40  is undergoing the orbital motion from the crank angle at which the first tier is opened to the crank angle at which the n-th tier is opened, at least the outermost one of the orbital-outer-surface contact points on the orbital scroll wrap  40   b  that is made of the material having the greater coefficient of linear expansion is out of contact with the fixed-inner-surface involute  30   c  at the orbital-outer-surface contact points. 
     If only the outermost contact point (i=3) has the relief  30   c   1 , the gap size δs at the innermost contact point (i=1) on the orbital-outer-surface involute cannot be made exactly 0 but can be made smaller than in the case where no relief  30   c   1  is provided. Therefore, some effect of strength improvement is produced. Providing the relief  30   c   1  only at the outermost contact point is regarded as limitedly providing the relief  30   c   1  in a region where the pressure difference is relatively small and refrigerant leakage is less likely to occur. Therefore, the occurrence of refrigerant leakage is suppressed, and high performance is achieved. The number of regions where the reliefs  30   c   1  are to be provided may be determined flexibly in consideration of the strength and performance required for the final product. 
     The effect of strength improvement that is produced in Embodiment 2 by providing the relief  30   c   1  is enhanced in the configuration according to Embodiment 1 in which the tiered bulbous shape of the scroll start portion satisfies the relationship of 0.3π&lt;φos(1)-φos(n)&lt;0.7π. Note that Embodiment 2 does not necessarily need to be applied to the configuration according to Embodiment 1. 
     According to Embodiment 2, the scroll compressor is configured to compress a working medium in the plurality of compression chambers  5   a  defined between the orbital scroll wrap  40   b  of the orbiting scroll  40  and the fixed scroll wrap  30   b  of the fixed scroll  30  that are made to mesh with each other. The working medium is compressed when the orbiting scroll  40  driven through the main shaft  7  undergoes an orbital motion about the fixed scroll  30 . The scroll compressor includes the variable crank mechanism that varies the radius of the orbital motion of the orbiting scroll  40 . The orbital scroll wrap  40   b  and the fixed scroll wrap  30   b  include respective scroll start portions each having a bulbous shape defined by connecting the involute start point of the outer-surface involute curve and the involute start point of the inner-surface involute curve to each other with a plurality of arcs. At least one of the scroll start portions has a tiered shape in which an n number (where n≥2) of tiers each having the bulbous shape are stacked in the axial direction of the main shaft  7 . The orbiting scroll  40  and the fixed scroll  30  are made of respective materials having different coefficients of linear expansion. The tiers of the scroll start portion are defined as the first tier, the second tier, . . . , and the n-th tier in order from the tip toward the base of the scroll start portion. The situation where the orbital scroll wrap  40   b  and the fixed scroll wrap  30   b  go out of contact with each other in the n-th tier of the scroll start portion and, among the compression chambers, two compression chambers that are not made to communicate with each other before the scroll wraps  40   b  and  30   b  go out of contact with each other are made to communicate with each other is expressed as the n-th tier is opened. The relief  30   c   1  is provided in the fixed scroll wrap  30   b  such that while the orbiting scroll  40  is undergoing the orbital motion from the crank angle at which the first tier is opened to the crank angle at which the n-th tier is opened, the outer-surface involute of the scroll wrap of the scroll that is made of the material having the greater coefficient of linear expansion and the inner-surface involute of the scroll wrap of the scroll that is made of the material having the smaller coefficient of linear expansion are out of contact with each other at least at the outermost one of the plurality of contact points where the two involutes are to come into contact with each other. 
     In such a configuration, the scroll start portion of the scroll wrap made of the material having the greater coefficient of linear expansion is kept supported by the lateral face of the scroll wrap of the scroll made of the material having the smaller coefficient of linear expansion until the pressure is completely equalized between two of the compression chambers, namely the innermost compression chamber  5   a   1  and the second compression chamber  5   a   2  that are not made to communicate with each other before the scroll wraps go out of contact with each other. Such a configuration suppresses the generation of a great stress at the base of the scroll start portion of the scroll wrap made of the material having the greater coefficient of linear expansion. Thus, an effect of strength improvement for the scroll start portion is produced. Furthermore, the scroll start portion of the scroll wrap made of the material having the smaller coefficient of linear expansion is designed such that, during operation, the gaps from the other scroll wrap made of the material having the greater coefficient of linear expansion become smaller than in the case where no relief  30   c   1  is provided. Such a configuration suppresses the generation of a great stress at the base of the scroll start portion. Thus, an effect of strength improvement for the scroll start portion is produced. 
     Embodiment 3 
     Embodiment 3 will now be described, except some of the features that are the same as those described in Embodiment 1 or 2. 
       FIG.  30    schematically illustrates a lateral section of a compression unit of a scroll compressor according to Embodiment 3. 
     In Embodiment 2 described above, the fixed-inner-surface involute  30   c  has the reliefs  30   c   1 . In Embodiment 3, the orbital-outer-surface involute  40   d  has reliefs  40   d   1 . 
     The regions where the reliefs  40   d   1  are to be provided are defined by the following seven parameters: 
     the involute angle at the start point,  40   d   1   a , of each relief  40   d   1 : φoa4; 
     the involute angle at the end point,  40   d   1   b , of each relief  40   d   1 : φob4; 
     the involute angle at the involute end point,  30   c   2 , of the fixed-inner-surface involute  30   c : φie3; 
     the involute-start-point angle of the inner-surface involute in the n-th tier of the fixed scroll wrap  30   b : φis3(n); 
     the involute-start-point angle of the outer-surface involute in the first tier of the orbital scroll wrap  40   b : φos4(1); 
     the involute-start-point angle of the outer-surface involute in the n-th tier of the orbital scroll wrap  40   b : φos4(n); and 
     the number of contact points between the orbital-outer-surface involute  40   d  and the fixed-inner-surface involute  30   c : m=[(φie3−φis3(n))/2π], where [ ] indicates that the decimals are to be rounded down. 
     Letting the serial numbers of the contact points on the outer surface of the orbiting scroll be i (1, 2, 3, . . . , and m from the inner side), where m≥2, the reliefs  40   d   1  are provided in regions where the following relationships are satisfied: 
       “ i≥ 2”,“φ oa 4&lt;(φ os 1( n ))+2π×( i− 1)”, and “φ ob 4&gt;(φ os 1(1))+2π×( i− 1)”.
 
     Specifically, the reliefs  40   d   1  are provided such that when the crank angle is between the involute-start-point angle φos(1) of the orbital-outer-surface involute  40   d  in the first tier and the involute-start-point angle φos(n) of the orbital-outer-surface involute  40   d  in the n-th tier, the orbital-outer-surface involute  40   d  and the fixed-inner-surface involute  30   c  are out of contact with each other at the orbital-outer-surface contact points expressed as i=2 or greater. In other words, the reliefs  40   d   1  are provided in the orbital-outer-surface involute  40   d  such that while the orbiting scroll  40  is undergoing the orbital motion from the crank angle at which the first tier is opened to the crank angle at which the n-th tier is opened, the orbital-outer-surface contact points, excluding the innermost one, on the orbital scroll wrap  40   b  that is made of the material having the greater coefficient of linear expansion are out of contact with the fixed-inner-surface involute  30   c  at the orbital-outer-surface contact points. 
     In Embodiment 3, the relief  40   d   1  is provided at each of all the contact points on the orbital-outer-surface involute but the innermost one. Alternatively, the relief  40   d   1  may be provided only at the outermost contact point (i=3). Specifically, the relief  40   d   1  only needs to be provided in the orbital-outer-surface involute  40   d  such that while the orbiting scroll  40  is undergoing the orbital motion from the crank angle at which the first tier is opened to the crank angle at which the n-th tier is opened, at least the outermost one of the orbital-outer-surface contact points on the orbital scroll wrap  40   b  that is made of the material having the greater coefficient of linear expansion is out of contact with the fixed-inner-surface involute  30   c  at the orbital-outer-surface contact points. 
     The orbiting scroll  40  and the fixed scroll  30  according to Embodiment 3 are made of respective materials having different coefficients of linear expansion. The tiers of the scroll start portion are defined as the first tier, the second tier, . . . , and the n-th tier in order from the tip toward the base of the scroll start portion. The situation where the orbital scroll wrap  40   b  and the fixed scroll wrap  30   b  go out of contact with each other in the n-th tier of the scroll start portion and the two compression chambers that are not made to communicate with each other before the scroll wraps  40   b  and  30   b  go out of contact with each other are made to communicate with each other is expressed as the n-th tier is opened. The relief  40   d   1  is provided in the orbital scroll wrap  40   b  such that while the orbiting scroll  40  is undergoing the orbital motion from the crank angle at which the first tier is opened to the crank angle at which the n-th tier is opened, the outer-surface involute of the scroll wrap of one of the scrolls that is made of the material having the greater coefficient of linear expansion and the inner-surface involute of the counterpart scroll wrap are out of contact with each other at least at the outermost one of the plurality of contact points where the two involutes are to come into contact with each other. 
     The effects produced by Embodiment 3 are the same as those produced by Embodiment 2.