Patent Publication Number: US-RE46106-E

Title: Scroll compressor

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Pursuant to 35 U.S.C. §119(a), this application This application is a reissue application of U.S. Pat. No. 8,308,460 issued on Nov. 13, 2012, which claims the benefit of the earlier filing date and the right of priority to Korean Applications No. 10-2011-0021108, filed on Mar. 9, 2011, and 10-2011-0046492, filed on May 17, 2011 the contents of which is incorporated by reference herein in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention This invention relates to a scroll compressor, and more particularly, to a configuration of a fixed scroll and an orbiting scroll of the scroll compressor capable of obtaining a sufficient compression ratio. 
     2. Background of the Invention 
     A scroll compressor is a compressor which includes a fixed scroll having a fixed wrap and an orbiting scroll having an orbiting wrap engaged with the fixed wrap. In this configuration of the scroll compressor, as the orbiting scroll orbits on the fixed scroll, the volumes of compression chambers, which are formed between the fixed wrap and the orbiting wrap, consecutively change, thereby sucking and compressing a refrigerant. 
     The scroll compressor allows suction, compression and discharge to be consecutively performed, so it is very favorable, as compared to other types of compressors, in the aspect of vibration and noise generated during operation. 
     The behavior of the scroll compressor may be dependent on the shapes of the fixed wrap and the orbiting wrap. The fixed wrap and the orbiting wrap may have a random shape, but typically they have a shape of an involute curve, which is easy to manufacture. The involute curve refers to a curve corresponding to a track drawn by an end of a thread when unwinding the thread wound around a basic circle with a predetermined radius. When such an involute curve is used, the wrap has a uniform thickness, and a rate of volume change of the compression chamber in response to a rotated angle of the orbiting scroll is constantly maintained. Hence, the number of turns of the wrap should increase to obtain a sufficient compression ratio, which may, however, cause the compressor to be increased in size corresponding to the increased number of turns of the wrap. 
     The orbiting scroll typically includes a disk, and the orbiting wrap is located at one side of the disk. A boss is formed at a rear surface of the disk opposite to the side at which the orbiting wrap is formed. The boss is connected to a rotation shaft, which allows the orbiting scroll to perform an orbiting motion. Such an arrangement with the orbiting wrap on one side of the disk and the boss on the other side of the disk allows the orbiting wrap to be formed on almost an entire surface of the disk, thereby reducing a diameter of the disk for obtaining a particular compression ratio. However, a point of application of a driving force at the boss which is opposed to a force of a refrigerant upon compression between the fixed wrap and the orbiting wrap is perpendicularly spaced apart from the wraps. Because the boss is not in the same plane on the same surface as the orbiting wrap, the orbiting scroll is inclined during operation, thereby generating more vibration and noise. 
     SUMMARY OF THE INVENTION 
     To overcome the drawbacks of the background art, a scroll compressor is provided that is capable of reducing an entire size of the compressor while ensuring a sufficient compression ratio. The orbiting scroll of the present invention is configured so that the orbiting wrap and the coupling portion for the rotation shaft are located at the same surface in the same plane. This arrangement allows the repulsive force of the refrigerant and the reaction force to be applied in the same plane so as to solve the inclination problem of the orbiting scroll of the background art. 
     Because the rotation shaft extends up to the orbiting wrap, an end portion of the rotation shaft is located in the central portion of the orbiting wrap, which has been used as a compression chamber in the background art. Therefore, to obtain a sufficient compression ratio, the fixed wrap and the orbiting wrap are uniquely configured. 
     In one exemplary embodiment, a scroll compressor includes a fixed scroll having a fixed wrap, an orbiting scroll having an orbiting wrap, the orbiting wrap configured to define first and second compression chambers in an outer side surface and an inner side surface together with the fixed wrap, the orbiting scroll performing an orbiting motion with respect to the fixed scroll, a rotation shaft having an eccentric portion at one end thereof, the eccentric portion coupled to the orbiting wrap to overlap with each other in a lateral direction, and a driving unit configured to drive the rotation shaft. 
     In accordance with one aspect of the invention, the first compression chamber is defined between two contact points P 1  and P 2  generated by the contact of an inner side surface of the fixed wrap and an outer side surface of the orbiting wrap, wherein α&lt;360° at least before initiating a discharge operation if a greater angle of angles defined by two lines, which connect a center O the eccentric portion to the two contact points P 1  and P 2 , respectively, is α. 
     In addition, l&gt;0 if a distance between normal lines at the two contact points P 1  and P 2  is 1. Also, the normal lines drawn at the two contact points P 1  and P 2  may be different from each other. 
     A rotation shaft coupling portion may be formed through a central portion of the orbiting scroll. The rotation shaft coupling portion may have an outer circumferential surface defining a part of the orbiting wrap and be coupled with the eccentric portion inside thereof. If the first compression chamber is located at the outer circumferential surface of the rotation shaft coupling portion, α&lt;360° and l&gt;0. 
     The second compression chamber may contact the outer circumferential surface of the rotation shaft coupling portion with moving internally along an inner circumferential surface of the orbiting wrap and then communicate with the first compression chamber. 
     The rotation shaft may include a shaft portion connected to the driving unit, a pin portion formed at an end of the shaft portion to be concentric with the shaft portion, and an eccentric bearing eccentrically inserted in the pin portion. The eccentric bearing may be rotatably coupled to the rotation shaft coupling portion. The pin portion may be formed to be asymmetric. 
     In accordance with another aspect of the invention, if an inner contact point of the first compression chamber upon initiation of discharging is P 3  and an inner contact point of the first compression chamber 150° before initiating the discharge operation is P 4 , a thickness of the fixed wrap is decreased and then increased as moving from P 3  to P 4 . The fixed wrap may have the maximum thickness between P 3  and an inner end portion of the fixed wrap. 
     In accordance with another aspect of the invention, if a distance between an inner circumferential surface of the fixed wrap and a shaft center of the rotation shaft is D F , an inner contact point of the first compression chamber upon initiation of discharging is P 3  and an inner contact point of the first compression chamber 150° before initiating the discharge operation is P 4 , the distance D F  is increased and then decreased. 
     In accordance with another aspect of the invention, if a distance between a center of the eccentric portion and an outer circumferential surface of the orbiting wrap is D O , an inner contact point of the first compression chamber upon initiation of discharging is P 3  and an inner contact point of the first compression chamber 150° before initiating the discharge operation is P 4 , the distance D O  is increased and then decreased as moving from P 3  to P 4 . 
     In accordance with another aspect of the invention, a rotation shaft coupling portion is formed in a central portion of the orbiting scroll, the eccentric portion coupled to the rotation shaft coupling portion, wherein a protruding portion protrudes from an inner circumferential surface of an inner end of the fixed wrap, and a recess portion is recessed at an outer circumferential surface of the rotation shaft coupling portion, the recess portion contacting at least part of the protruding portion. 
     In accordance with another aspect of the invention, a rotation shaft coupling portion is formed at a central portion of the orbiting scroll, the rotation shaft coupling portion having an outer circumferential surface configuring a part of the orbiting wrap and having the eccentric portion coupled therein, wherein if an inner contact point of the first compression chamber upon initiation of discharging is P 3  and an inner contact point of the first compression chamber 90° prior to initiation of discharging is P 5 , R m  defined by the following equation is smaller than an inner radius R H  of the rotation shaft coupling portion at an interval between P 3  and P 5 : 
               R   m     =       1   90     ⁢       ∫   0   90     ⁢       R   θ     ⁢           ⁢     ⅆ   θ                 
where R 74  is a radius of curvature of the orbiting wrap at the inner contact point of the first compression chamber when a rotation angle of the rotation shaft is θ. Here, R m  may be smaller than R H /1.4, and in more detail, R m  may be smaller than 10.5 mm.
 
     In accordance with another aspect of the invention, if an inner contact point of the first compression chamber upon initiation of discharging is P 3 , a shortest distance between a tangent line at P 3  and a center O of the eccentric portion is smaller than a diameter radius of the eccentric portion. 
     In accordance with these aspects of the invention, the compression ratio of the first compression chamber can be increased as compared to a scroll compressor having a fixed wrap and an orbiting wrap having an involute shape. In addition, as a thickness of an inner end portion of the fixed wrap varies, wrap rigidity can be enhanced and leakage prevention capability can be improved. 
     Further scope of applicability of the present application will become more apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating particular embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate exemplary embodiments and together with the description serve to explain the principles of the invention. 
         FIG. 1  is a sectional view schematically showing an inner structure of a scroll compressor in accordance with one exemplary embodiment. 
         FIG. 2  is a partially cut-away view showing a compression unit of the exemplary embodiment shown in  FIG. 1 . 
         FIG. 3  is a disassembled perspective view of the compression unit shown in  FIG. 2 . 
         FIGS. 4(a) and 4(b)  are schematic views showing first and second compression chambers right after suction and right before discharge in a scroll compressor having an orbiting wrap and a fixed wrap in the involute shape. 
         FIG. 5  is a planar schematic view showing an orbiting wrap with an involute shape. 
         FIGS. 6(a)-6(e)  are views showing a process for obtaining generating curves in the scroll compressor of the one exemplary embodiment. 
         FIG. 7  is a planar view showing the final generating curves shown in  FIGS. 6(a)-6(e) . 
         FIG. 8  is a planar view showing an orbiting wrap and a fixed wrap formed by the generating curve shown in  FIG. 7 . 
         FIG. 9  is an enlarged planar view of a central portion of  FIG. 8 . 
         FIG. 10  is a graph showing a relationship between an angle α and a compression ratio. 
         FIG. 11  is a planar view showing a state that the orbiting wrap contacts with the fixed wrap at point P 3 . 
         FIG. 12  is a planar view showing a state that the orbiting wrap contacts with the fixed wrap at point P 5 . 
         FIGS. 13(a) and 13(b)  are schematic sectional views showing embodiments of a rotation shaft coupling portion of the orbiting scroll. 
         FIG. 14  is a graph showing changes of compression ratios in response to an average radius of curvature R m  in the exemplary embodiment of  FIG. 8 . 
         FIG. 15  is a planar view showing a state that the orbiting wrap contacts with the fixed wrap at point P 4 . 
         FIG. 16  is a planar view showing a time point when initiating a discharge operation in a second compression chamber in the exemplary embodiment of  FIG. 8 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, description will be made in detail to the exemplary embodiments of a scroll compressor according to this invention with reference to the accompanying drawings. 
     As shown in  FIG. 1 , the exemplary embodiment may include a hermetic compressor  100  having a cylindrical casing  110 , and an upper shell  112  and a lower shell  114  for covering upper and lower portions of the casing  110 . The upper and lower shells  112  and  114  may be welded to the casing  110  so as to define a single hermetic space together with the casing  110 . A lower space of the hermetic compressor  100  may define a suction space, and an upper space thereof may define a discharging space. The lower and upper spaces may be divided based upon an upper frame  115  to be explained later. 
     A discharge pipe  116  may be connected to an upper side of the upper shell  112 . The discharge pipe  116  may act as a path through which a compressed refrigerant is discharged to the outside. An oil separator (not shown) for separating oil mixed with the discharged refrigerant may be connected to the discharge pipe  116 . A suction pipe  118  may be installed at a side surface of the casing  110 . The suction pipe  118  may act as a path through which a refrigerant to be compressed is introduced. Referring to  FIG. 1 , the suction pipe  118  is located at an interface between the casing  110  and the upper shell  116 , but the position of the suction pipe  118  is not limited to this example. In addition, the lower shell  114  may function as an oil chamber for storing oil, which is supplied to make the compressor work smoothly. 
     A motor  120  as a driving unit may be installed at an approximately central portion within the casing  110 . The motor  120  may include a stator  122  fixed to an inner surface of the casing  110 , and a rotor  124  located within the stator  122  and rotatable by interaction with the stator  122 . A rotation shaft  126  may be disposed in the center of the rotor  124  so as to be rotatable together with the rotor  124 . 
     An oil passage  126 a may be formed in the center of the rotation shaft  126  along a lengthwise direction of the rotation shaft  126 . An oil pump  126 b for pumping up oil stored in the lower shell  114  may be installed at a lower end portion of the rotation shaft  126 . The oil pump  126 b may be implemented by forming a spiral recess or separately installing an impeller in the oil passage  126 a, or may be a separately welded pump. 
     A diameter-extended part  126 c, which is inserted in a boss formed in a fixed scroll to be explained later, may be disposed at an upper end portion of the rotation shaft  126 . The diameter-extended part  126 c may have a diameter greater than other parts. A pin portion  126 d may be formed at an end of the diameter-extended part  126 c. Alternatively, the diameter-extended part  126 c may not be utilized, and the entire rotation shaft  126  may have a specific diameter. 
     An eccentric bearing  128  may be inserted on the pin portion  126 d, as shown in  FIG. 2 . Referring to  FIG. 3 , the eccentric bearing  128  may eccentrically be inserted on the pin portion  126 d. A coupled portion between the pin portion  126 d and the eccentric bearing  128  may have a shape like the letter “D” such that the eccentric bearing  128  cannot be rotated with respect to the pin portion  126 d. 
     A fixed scroll  130  may be mounted at a boundary portion between the casing  110  and the upper shell  112 . The fixed scroll  130  may have an outer circumferential surface, which is shrink-fitted between the casing  110  and the upper shell  112 . Alternatively, the fixed scroll  130  may be welded with the casing  110  and the upper shell  112 . 
     A boss  132 , in which the rotation shaft  126  is inserted, may be formed at a lower surface of the fixed scroll  130 . A through hole through which the pin portion  126 d of the rotation shaft  126  is inserted, may be formed through an upper surface of the boss  132 , as shown in  FIG. 1 . Accordingly, the pin portion  126 d can protrude to an upper side of a disk  134  of the fixed scroll  130  through the through hole. 
     A fixed wrap  136 , which is engaged with an orbiting wrap to be explained later so as to define compression chambers, may be formed at an upper surface of the disk  134 . A side wall  138  may be located at an outer circumferential portion of the disk  134 . The side wall  138  may define a space for housing an orbiting scroll  140  to be explained later and be contactable with an inner circumferential surface of the casing  110 . An orbiting scroll support  138 a, on which an outer circumferential portion of the orbiting scroll  140  is received, may be formed inside an upper end portion of the side wall  138 . A height of the orbiting scroll support  138 a may have the same height as the fixed wrap  136  or be slightly lower than the fixed wrap  136 , such that an end of the orbiting wrap can contact a surface of the disk  134  of the fixed scroll  130 . 
     The orbiting scroll  140  may be disposed on the fixed scroll  130 . The orbiting scroll  140  may include a disk  142  having an approximately circular shape and an orbiting wrap  144  engaged with the fixed wrap  136 . A rotation shaft coupling portion  146  in an approximately circular shape may be formed into the central portion of the disk  142  such that the eccentric bearing  128  can be rotatably inserted therein. An outer circumferential portion of the rotation shaft coupling portion  146  may be connected to the orbiting wrap  144  so as to define compression chambers together with the fixed wrap  136  during compression, which will be described later. 
     The eccentric bearing  128  may be inserted into the rotation shaft coupling portion  146 , and the end portion of the rotation shaft  126  may be inserted through the disk  134  of the fixed scroll  130 , so that the orbiting wrap  144 , the fixed wrap  136  and the eccentric bearing  128  may overlap in a lateral direction of the compressor. Upon compression, a repulsive force of a refrigerant may be applied to the fixed wrap  136  and the orbiting wrap  144 , while a compression force as a reaction force against the repulsive force may be applied between the rotation shaft coupling portion  146  and the eccentric bearing  128 . As such, when the shaft is partially inserted through the disk and overlaps with the wrap, the repulsive force of the refrigerant and the compression force may be applied to the same side surface based on the disk, thereby being attenuated by each other. Consequently, the orbiting scroll  140  can be obviated from being inclined due to the compression force and the repulsive force. As alternate example, an eccentric bush may be installed instead of the eccentric bearing. In this example, an inner surface of the rotation shaft coupling portion  146 , in which the eccentric bush is inserted, may be specifically processed to serve as a bearing. Also, another example of installing a separate bearing between the eccentric bush and the rotation shaft coupling portion may be conceived. 
     A discharge hole  140 a may be formed at the disk  142  such that a compressed refrigerant can be discharged into the casing. The position and shape of the discharge hole  140 a may be determined by considering a required discharge pressure or the like. The disk  142  may further include a bypass hole in addition to the discharge hole  140 a. When the bypass hole is farther away from the center of the disk  142  than the discharge hole  140 a, the bypass hole may have a diameter greater than one third of an effective diameter of the discharge hole  140 a. 
     An Oldham ring  150  for preventing rotation of the orbiting scroll  140  may be installed on the orbiting scroll  140 . The Oldham ring  150  may include a ring part  152  having an approximately circular shape and inserted on a rear surface of the disk  142  of the orbiting scroll  140 , and a pair of first keys  154  and a pair of second keys  156  protruding to one side surface of the ring part  152 . The first keys  154  may protrude longer than a thickness of an outer circumferential portion of the disk  142  of the orbiting scroll  140 , thereby being inserted into first key recesses  154 a, which are recessed over an upper end of the side wall  138  of the fixed scroll  130  and the orbiting scroll support  138 a. In addition, the second keys  156  may be inserted into second key recesses  156 a, which are formed at the outer circumferential portion of the disk  142  of the orbiting scroll  140 . 
     Each of the first key recesses  154 a may have a perpendicular portion extending upwardly and a horizontal portion extending in a right-and-left direction. During an orbiting motion of the orbiting scroll  140 , a lower end portion of each first key  154  remains inserted in the horizontal portion of the corresponding first key recess  154 a while an outer end portion of the first key  154  in a radial direction is separated from the perpendicular portion of the first key recess  154 a. That is, the first key recesses  154 a and the fixed scroll  130  are coupled to each other in a perpendicular direction, which may allow reduction of a diameter of the fixed scroll  130 . 
     In detail, a clearance (air gap) as wide as an orbiting radius should be ensured between the disk  142  of the orbiting scroll  140  and an inner wall of the fixed scroll  130 . If the keys of an Oldham ring are coupled to a fixed scroll in a radial direction, key recesses formed at the fixed scroll should be longer than at least the orbiting radius in order to prevent the Oldham ring from being separated from the key recesses during orbiting motion. However, this structure may cause an increase in the size of the fixed scroll. 
     On the other hand, as shown in the exemplary embodiment, if the second key recess  156 a extend down to a lower side of a space between the disk  142  of the orbiting scroll  140  and the orbiting wrap  144 , a sufficient length of the key recess  156 a can be ensured even without increasing the size of the fixed scroll  130 . 
     In addition, in the exemplary embodiment, all the keys of the Oldham ring  150  are formed at the one side surface of the ring part  152 . This structure can thus reduce the perpendicular height of a compression unit as compared to forming keys at both side surfaces. 
     Meanwhile, as shown in  FIG. 1 , a lower frame  113  for rotatably supporting a lower side of the rotation shaft  126  may be installed at a lower side of the casing  110 , and an upper frame  115  for supporting the orbiting scroll  140  and the Oldham ring  150  may be installed on the orbiting scroll  140 . A hole  115 a is formed in the upper frame  115 . The hole  115 a may communicate with a discharge hole  140 a of the orbiting scroll  140  to allow a compressed refrigerant to be discharged therethrough toward the upper shell  112 . 
     Hereinafter, prior to explaining the shape of a fixed scroll and an orbiting scroll of the present invention, a description will be given of an example with an orbiting wrap and a fixed wrap each having an involute form to help understanding the invention. 
       FIGS. 4(a) and 4(b)  are planar views showing a compression chamber right after a suction operation and a compression chamber right before a discharge operation in a scroll compressor having an orbiting wrap and a fixed wrap formed as an involute curve and having a shaft partially inserted through a disk.  FIG. 4(a)  shows the change of a first compression chamber defined between an inner side surface of the fixed wrap and an outer side surface of the orbiting wrap, and  FIG. 4(b)  shows the change of a second compression chamber defined between an inner side surface of the orbiting wrap and an outer side surface of the fixed wrap. 
     In the configuration of a scroll compressor, a compression chamber is defined between two contact points generated by contact between the fixed wrap and the orbiting wrap. Upon having the fixed wrap and the orbiting wrap having an involute curve, as shown in  FIGS. 4(a) and 4(b) , two contact points defining one compression chamber are present on a line. In other words, the compression chamber extends 360° with respect to the center of the rotation shaft. 
     Regarding a volume change of the first compression chamber shown in  FIG. 4(a) , the volume of the compression chamber is gradually reduced moving toward the central portion in response to the orbiting motion of the orbiting scroll. Thus, when arriving at an outer circumferential portion of a rotation shaft coupling portion located at the center of the orbiting scroll, the first compression chamber has the minimum volume value. For the fixed wrap and the orbiting wrap having the involute curve, the volume reduction rate linearly decreases as an orbiting angle (hereinafter, referred to as ‘crank angle’) of the rotation shaft increases. Hence, to acquire a high compression ratio, the compression chamber should move as close as possible toward the center. However, when the rotation shaft is present at the central portion, the compression chamber may only move inward to the outer circumferential portion of the rotation shaft. Accordingly, the compression ratio is lowered. A compression ratio of about 2.13:1 is exhibited in  FIG. 4(a) . 
     Meanwhile, the second compression chamber shown in  FIG. 4(b)  has a much lower compression ratio than the first compression chamber, being about 1.46:1. However, regarding the second compression chamber, if the shape of the orbiting scroll is changed such that a connected portion between a rotation shall coupling portion P and the orbiting wrap is formed in an arcuate shape, a compression path of the second compression chamber until before a discharge operation extends, thereby increasing the compression ratio up to about 3.0. In this case, the second compression chamber may extend less than 360° right before the discharge operation. However, this method may not be applied to the first compression chamber. 
     Therefore, when the fixed wrap and the orbiting wrap have the involute shape, the second compression chamber may have a high compression ratio but the first compression chamber may not. Also, when the two compression chambers have a remarkable difference of their compression ratios, it may badly affect the operation of the compressor and even may lower the overall compression ratio. 
     To solve the problem, the exemplary embodiment shows the fixed wrap and the orbiting wrap having a different curve (shape) from the involute curve.  FIGS. 6(a)-6(e)  show a process of deciding shapes of the fixed wrap and the orbiting wrap according to the exemplary embodiment. In  FIGS. 6(a)-6(e) , a solid line indicates a generating curve for the first compression chamber and a dotted line indicates a generating curve for the second compression chamber. 
     Here, the generating curve refers to a track drawn by a particular shape during movement. The solid line indicates a track drawn by the first compression chamber during suction and discharge operations, and the dotted line indicates the track of the second compression chamber. Hence, if the generating curve is moved in parallel to both sides as long as the orbiting radius of the orbiting scroll based upon the solid line, it exhibits the shapes of an inner side surface of the fixed wrap and an outer side surface of the orbiting wrap. If the generating curve is moved in parallel based upon the dotted line, it exhibits the shapes of an outer side surface of the fixed wrap and an inner side surface of the orbiting wrap. 
       FIG. 6(a)  shows a generating curve corresponding to having the wrap shape shown in  FIG. 5 . Here, a part indicated by a bold line corresponds to the first compression chamber right before a discharge operation. As shown, a start point and an end point are present on a line. In this case, it is difficult to obtain a sufficient compression ratio. Thus, as shown in  FIG. 6(b) , an end portion of the bold line, located outside, is transferred in a clockwise direction along the generating curve and an end portion located inside is transferred up to a point to be contactable with the rotation shaft coupling portion. That is, a portion of the generating curve, adjacent to the rotation shaft coupling portion, may be curved to have a smaller radius of curvature. 
     As described above, in the aspect of the characteristic of the scroll compressor, the compression chamber is formed by two contact points where the orbiting wrap and the fixed wrap contact each other. Both ends of the bold line in  FIG. 6(a)  correspond to the two contact points. Normal vectors at the respective contact points are in parallel to each other according to the operating algorithm of the scroll compressor. Also, the normal vectors are in parallel to a line connecting a center of the rotation shaft and a center of the eccentric bearing. Here, for the fixed wrap and the orbiting wrap having the involute shape, the two normal vectors are in parallel to each other and also present on the same line as shown in  FIG. 6(a) . 
     In  FIG. 6(a) , if it is assumed that the center of the rotation shaft coupling portion  146  is O and two contact points are P 1  and P 2 , P 2  is located on a line connecting O and P 1 . If it is assumed that a larger angle of angles formed by lines OP 1  and OP 2  is α, α is 360°. In addition, if it is assumed that a distance between the normal vectors at P 1  and P 2  is l, l is 0. 
     The inventors have observed from the research that when P 1  and P 2  are transferred more internally along the generating curves, the compression ratio of the first compression chamber can be improved. To this end, when P 1  is transferred toward the rotation shaft coupling portion  146 , namely, the generating curve for the first compression chamber is transferred by turning toward the rotation shaft coupling portion  146 , P 1 , which has the normal vector in parallel to the normal vector at P 2 , then rotates in a clockwise direction based on  FIG. 6(b) , as compared to  FIG. 6(a) , thereby being located at the rotated point. As described above, the first compression chamber is reduced in volume by being transferred more internally along the generating curve. Hence, the first compression chamber shown in  FIG. 6(b)  may be transferred more internally as compared to  FIG. 6(a) , and further compressed as much as being transferred, thereby obtaining an increased compression ratio. 
     Referring to  FIG. 6(b) , the point P 1  is excessively close to the rotation shaft coupling portion  146 , and therefore the rotation shaft coupling portion  146  becomes thinner in thickness. Hence, the point P 1  is transferred back so as to modify the generating curve as shown in  FIG. 6(c) . Here, in  FIG. 6(c) , the generating curves of the first and second compression chambers are excessively close to each other, which makes a wrap thickness too thin or prevents a wrap from being physically formed. Thus, as shown in  FIG. 6(d) , the generating curve of the second compression chamber may be modified such that the two generating curves can maintain a predetermined interval therebetween. 
     Furthermore, the generating curve of the second compression chamber is modified, as shown in  FIG. 6(e) , such that an arcuate portion A located at the end of the generating curve of the second compression chamber is contactable with the generating curve of the first compression chamber. The generating curves may be modified to continuously maintain a predetermined interval therebetween. When a radius of the arcuate portion A of the generating curve of the second compression chamber is increased to ensure a wrap rigidity at the end of the fixed wrap, generating curves having the shape shown in  FIG. 7  may be obtained. 
       FIG. 8  is a planar view showing an orbiting wrap and a fixed wrap obtained based on the generating curves of  FIG. 7 , and  FIG. 9  is an enlarged planar view of the central portion of  FIG. 8 . For reference,  FIG. 8  shows a position of the orbiting wrap at a time point of initiating the discharge operation in the first compression chamber. Here, the point P 1  in  FIG. 8  indicates a point, which is present inside, of two contact points defining a compression chamber, at the moment when initiating discharging in the first compressor chamber. Line S is a virtual line for indicating a position of the rotation shaft and circle C is a track drawn by the line S. Hereinafter, the crank angle is set to 0° when the line S is present in a state shown in  FIG. 8 , namely, when initiating discharging, set to a negative (−) value when rotated counterclockwise, and set to a positive (+) value when rotated clockwise. 
     Referring to  FIGS. 8 and 9 , it can be exhibited that an angle α defined by two lines, which connect the two contact points P 1  and P 2  respectively to the center O of the rotation shaft coupling portion is smaller than 360°, and a distance l between the normal vectors at each of the contact points P 1  and P 2  is greater than 0. Accordingly, the first compression chamber right before a discharge operation can have a smaller volume than that defined by the fixed wrap and the orbiting wrap having the involute shape, which results in an increase in the compression ratio. In addition, the orbiting wrap and the fixed wrap shown in  FIG. 8  have a shape that a plurality of arcs having different diameters and origins are connected and the outermost curve may have an approximately oval shape with a major axis and a minor axis. 
     In the exemplary embodiment, the angle α may be set to have a value in the range of 270° to 345°.  FIG. 10  is a graph showing the angle α and a compression ratio. From the perspective of improvement of a compression ratio, it may be advantageous to set the angle α to have a low value. However, if the angle α is smaller than 270°, it may inhibit mechanical processing, thereby deriving bad productivity and increasing a price of a compressor. If the angle α exceeds 345°, the compression ratio may be lowered below 2.1, thereby failing to provide a sufficient compression ratio. 
     In addition, a protruding portion  160  may protrude from near an inner end of the fixed wrap toward the rotation shaft coupling portion  146 . A contact portion  162  may further be formed by protruding from the protruding portion  160 . That is, the inner end of the fixed wrap  130  may be thicker than other portions. Accordingly, the wrap rigidity of the inner end of the fixed wrap, to which the strongest compression force is applied, can be improved, resulting in enhanced durability. 
     The thickness of the fixed wrap is gradually decreased, starting from the inner contact point P 1  of the two contact points defining the first compression chamber upon initiating the discharge operation, as shown in  FIG. 9 . More particularly, a first part  164  may be formed adjacent to the contact point P 1  and a second part  166  may extend from the first part  164 . A thickness reduction rate at the first part  164  may be higher than that at the second part  166 . After the second part  166 , the fixed wrap may be increased in thickness within a predetermined interval. 
     If it is assumed that a distance between an inner side surface of the fixed wrap and a center O′ of the rotation shaft is D F , the distance D F  may be increased and then decreased moving away from P 1  in a counterclockwise direction (based on  FIG. 9 ), and such interval is shown in  FIG. 15 .  FIG. 15  is a planar view showing the position of the orbiting wrap 150° before initiating the discharge operation, namely, when the crank angle is 210°. If the rotation shaft rotates 150° more from the state of  FIG. 15 , it reaches the state shown in  FIG. 9 . Referring to  FIG. 15 , an inner contact point P 4  of two contact points defining the first compression chamber is located above the rotation shaft coupling portion  146 , and the D F  is increased and then decreased at the interval from P 1  of  FIG. 9  to P 4  of  FIG. 15 . 
     The rotation shaft coupling portion  146  may be provided with a recess portion  170  engaged with the protruding portion  160 . One side wall of the recess portion  170  may contact the contact portion  162  of the protruding portion  160  to define one contact point of the first compression chamber. If it is assumed that a distance between the center O of the rotation shaft coupling portion  146  and an outer circumferential portion of the rotation shaft coupling portion  146  is D O , the distance D O  may be increased and then decreased along the interval between P 1  of  FIG. 9  and P 4  of  FIG. 15 . Similarly, the thickness of the rotation shaft coupling portion  146  may also be increased and then decreased along the interval between P 1  of  FIG. 9  and P 4  of  FIG. 15 . 
     The one side wall of the recess portion  170  may include a first increase part  172  at which a thickness is relatively greatly increased, and a second increase part  174  extending from the first increase part  172  and having a thickness increased at a relatively low rate. These correspond to the first part  164  and the second part  166  of the fixed wrap  136 . The first increase part  172 , the first part  164 , the second increase part  174  and the second part  166  may be obtained by turning the generating curve toward the rotation shaft coupling portion  146  at the step of  FIG. 6(b) . Accordingly, the inner contact point P 1  defining the first compression chamber may be located at the first and second increase parts  172 ,  174 , and also the length of the first compression chamber right before the discharge operation may be shortened so as to enhance the compression ratio. 
     Another side wall of the recess portion  170  may have an arcuate shape. A diameter of the arc may be determined based on the wrap thickness of the end of the fixed wrap  136  and the orbiting radius of the orbiting wrap  144 . When the thickness of the end of the fixed wrap increases, the diameter of the arc increases. Accordingly, the thickness of the orbiting wrap near the arc may increase to ensure durability, and the compression path may also extend so as to increase the compression ratio of the second compression chamber. 
     The central portion of the recess portion  170  may form a part of the second compression chamber.  FIG. 16  is a planar view showing the position of the orbiting wrap when initiating the discharge operation in the second compression chamber. Referring to  FIG. 16 , the second compression chamber is defined between two contact points P 6  and P 7  and contacts an arcuate side wall of the recess portion  170 . When the rotation shaft rotates more, one end of the second compression chamber may pass through the center of the recess portion  170 . 
       FIG. 11  is another planar view showing a state that is also shown in  FIG. 9 . Referring to  FIG. 11 , a tangent line T drawn at the point P 3 , which is the same as point P 1  of  FIG. 9 , passes through the inside of the rotation shaft coupling portion. This results from the behavior that the generating curve is curved inwardly during the process of  FIG. 6(b) . Consequently, a shortest distance between the tangent line T and a center of the rotation shaft coupling portion O is smaller than a radius R H  within the rotation shaft coupling portion, so that a shortest distance between the tangent line T at P 3  and a center O of the eccentric bearing  128  is smaller than a radius of the eccentric bearing  128 . 
     Referring to  FIGS. 13(a) and 13(b) , the inner radius R H  may be defined as an inner radius of the rotation shaft coupling portion when an inner circumferential surface of the rotation shaft coupling portion or an outer circumferential surface of the eccentric bearing is lubricated without a separate bearing, as shown in  FIG. 13(a) , or may be defined as an outer radius of the bearing when a separate bearing is additionally employed within the rotation shaft coupling portion as shown in  FIG. 13(b) . 
     In  FIGS. 11 and 12 , a point P 5  denotes an inner contact point when the crank angle is 270°, as shown in  FIG. 12 . A radius of curvature of an outer circumference of the rotation shaft coupling portion may have various values depending on each position between the points P 3  and P 5 . Here, the average radius of curvature R m  defined by the following equation may influence the compression ratio of the first compression chamber: 
               R   m     =       1   90     ⁢       ∫   0   90     ⁢       R   θ     ⁢           ⁢     ⅆ   θ                 
where R θ  is a radius of curvature of the orbiting wrap at the inner contact point of the first compression chamber when the crank angle is θ.
 
       FIG. 14  is a graph showing a relationship between an average radius of curvature and a compression chamber. In general, regarding a rotary compressor, it may have a compression ratio more than 2.3 when being used for both cooling and heating, and more than 2.1 when being used for cooling. Referring to  FIG. 14 , when the average radius of curvature is less than 10.5, the compression ratio may be more than 2.1. Therefore, if R m  is set to be less than 10.5 mm, the compression ratio may be more than 2.1. Here, R m  may be optionally set to be suitable for the use of the scroll compressor. In the exemplary embodiment, the R H  may have a value of approximately 15 mm. Therefore, the R m  may be set to be smaller than R H /1.4. 
     Meanwhile, the point P 5  may not always be limited to when the crank angle is 270°. In view of the operating algorithm of the scroll compressor, a design variable with respect to a radius of curvature up to 270° is low. Accordingly, in order to improve a compression ratio, it is advantageous to change a shape between 270° and 360°, in which the design variable is relatively high. 
     The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present disclosure. The present teachings can be readily applied to other types of apparatuses. This description is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. The features, structures, methods, and other characteristics of the exemplary embodiments described herein may be combined in various ways to obtain additional and/or alternative exemplary embodiments. 
     As the present features may be embodied in several forms without departing from the characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its scope as defined in the appended claims, and therefore all changes and modifications that fall within the metes and bounds of the claims, or equivalents of such metes and bounds, are therefore intended to be embraced by the appended claims.