Patent Publication Number: US-2006013711-A1

Title: Hermetic compresssor

Description:
TECHNICAL FIELD  
      The present invention relates to a hermetic compressor used in refrigeration cycle, such as refrigerator, air conditioner and freezer.  
     BACKGROUND ART  
      Recently, hermetic compressors used in refrigerators and freezers for household use are strongly demanded to be smaller in size, lower in noise and lower in vibration. In this background, the refrigerant is being shifted to hydrocarbon refrigerant which is natural refrigerant of low global warming coefficient represented by R600a noted for zero ozone depletion coefficient. Besides, to keep balance with the piston which is a main source of vibration, the method of using a balance weight is effective technology for reducing vibrations.  
      Hitherto, as this kind of hermetic compressor using balance weight, it is attempted to adjust the imbalanced force of the compressor mechanism by equipping a crankshaft with a balance weight of a nearly arc profile.  
      Referring now to the drawings, a conventional hermetic compressor disclosed in Japanese Laid-open Patent 2000-213462 is described below.  
       FIG. 5  is a longitudinal sectional view of the conventional compressor.  FIG. 6  is a plan sectional view of the conventional compressor.  
      In  FIG. 5  and  FIG. 6 , closed container  1  is filled with refrigerant  2 . Electric motor element  5  composed of stator  3  having winding  3   a  and rotor  4 , and compression element  6  driven by electric motor element  5  are elastically accommodated in container  1  by means of suspension spring  7 . Shaft  10  has main shaft body  11  press-fitting rotor  4  and eccentric shaft body  12  formed eccentrically to main shaft body  11 . Above eccentric shaft body  12 , balance weight  22  of which outer circumference is a nearly arc profile centered on the axial center of main shaft body  11  is fixed. Cylinder block  16  has nearly cylindrical compression chamber  17 . Piston  20  is inserted in compression chamber  17  so as to be freely slidable reciprocally. Piston  20  is coupled to eccentric shaft body  12  by means of connecting means  21 .  
      In the hermetic compressor having such configuration, the operation is described below.  
      Rotor  4  of electric motor element  5  rotates piston  20 . As rotary motion of eccentric shaft body  12  is transferred to piston  20  by way of connecting means  21 , piston  20  moves reciprocally in compression chamber  17 . As a result, refrigerant gas is sucked and compressed in compression chamber  17  from a cooling system (not shown), and discharged again into the cooling system.  
      At this time of compression action, as piston  20  makes reciprocal motions, reciprocal inertial force is generated as imbalanced force. This reciprocal inertial force is balanced by installing balance weight  22  so as to be in reverse phase to piston  20 . In this configuration, the reciprocal inertial force of piston  20  in horizontal direction is canceled to a certain extent.  
      In the conventional structure, to lower the overall height of the compressor, when balance weight  22  is disposed on a horizontal extension of piston  20 , balance weight  22  and piston  20  come to closest distance at the bottom dead center of piston  20 . To avoid such interference, balance weight  22  is designed in a nearly arc profile. Accordingly, balance weight  22  does not have sufficient inertial force. That is, reciprocal inertial force of piston  20  cannot be canceled sufficiently, and vibration of the hermetic compressor is increased.  
     DISCLOSURE OF THE INVENTION  
      The invention is devised in the light of the above problems of the prior art, and it is hence an object thereof to present a hermetic compressor of low vibration having a balance weight with a greater inertial force, in a configuration of disposing a balance weight on a horizontal extension of a piston.  
      The hermetic compressor of the invention comprises (i) an electric moter element, (ii) a compression element driven by the electric moter element, (iii) a closed container accommodating the electric moter element and compression element, and (iv) a refrigerant contained in the closed container. The compression element comprises (i) a shaft having an eccentric shaft body and a main shaft body, (ii) a cylinder block having a compression chamber, (iii) a piston moving reciprocally in the compression chamber, (iv) connecting means for connecting the piston and eccentric shaft body, and (v) a balance weight formed on the shaft. The piston is positioned on a horizontal extension of the balance weight. The outer circumference of the balance weight is formed in such a shape that the distance between the outer circumference of the balance weight and the piston is substantially constant along the closely approaching interval of the balance weight and piston. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a longitudinal sectional view of hermetic compressor in preferred embodiment of the invention.  
       FIG. 2  is a plan sectional view of hermetic compressor in the same preferred embodiment.  
       FIG. 3  is an essential magnified view of hermetic compressor in the same preferred embodiment.  
       FIG. 4  is an essential model diagram of hermetic compressor in the same preferred embodiment.  
       FIG. 5  is a longitudinal sectional view of a conventional compressor.  
       FIG. 6  is a plan sectional view of the conventional compressor. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
      A preferred embodiment of the invention is described specifically below while referring to the accompanying drawings.  
       FIG. 1  is a longitudinal sectional view of hermetic compressor in preferred embodiment of the invention.  FIG. 2  is a plan sectional view of the same preferred embodiment.  FIG. 3  is an essential magnified view of the same preferred embodiment.  FIG. 4  is an essential model diagram of the same preferred embodiment.  
      In  FIG. 1  to  FIG. 4 , closed container  101  is filled with refrigerant  102  composed of isobutane (R600a). Electric moter element  105  composed of stator  103  and rotor  104 , and compression element  106  driven by electric moter element  105  are elastically accommodated in closed container  101  by means of suspension spring  107 . Electric moter element  105  is driven by inverter at plural operating frequencies including an operating frequency of less than the power source frequency. Herein, a frequency of 30 Hz or less is included in the operating frequency. Closed container  101  is supported by grommet  126 .  
      Shaft  110  has (i) main shaft body  111  press-fitting rotor  104 , (ii) eccentric shaft body  112  formed eccentrically to main shaft body  111 , (iii) subsidiary shaft body  113  provided coaxially with main shaft body  111 , (iv) joint  114  for connecting between eccentric shaft body  112  and subsidiary shaft body  113 , and (v) balance weight  122  made of same material as shaft  110  in the lower part of subsidiary shaft body  113 . Piston  120  is positioned on a horizontal extension of balance weight  122 .  
      Cylinder block  116  having compression chamber  117  of nearly cylindrical shape has subsidiary bearing  119  for supporting subsidiary shaft body  113  above it. Beneath cylinder block  116 , main bearing  118  for supporting main shaft body  111  is fixed by means of screw  123 . Piston  120  is slidably inserted in compression chamber  117  of cylinder block  116 . Piston  120  is coupled with eccentric shaft body  112  by means of connecting means  121 . Supposing axial center Lila of main shaft body  111  to be origin, coordinates (x, y) of outer circumference of balance weight  122  are expressed as (expression-1) and (expression-2). 
 
 x=[s ·cos (360°−θ)+ L ·cos {(sin −1 ( s ·sin (360°−θ)/ L )}+ C−α]·cos ( 360°−θ)  (expression-1) 
 
 y=[s ·cos (360°−θ)+ L ·cos {(sin −1 ( s ·sin (360°−θ)/ L )}+ C−α]·sin ( 360°−θ)  (expression-2) 
 
 where s: eccentric amount of shaft  110  (distance between axial center  11   a  of main shaft body  111  and axial center  112   a  of eccentric shaft body  112 ) 
      L: pitch length of connecting means  121  
        C: skirt length of piston  120      α: distance between outer circumference of balance weight  122  and piston  120      θ: rotation angle of eccentric shaft body  112     
       

      For example, supposing eccentric amount s of shaft  110  to be 10 mm, pitch length L of connecting means  121  to be 37.3 mm, skirt length C of piston  120  to be 9.9 mm, and distance a between outer circumference of balance weight  122  and piston  120  to be 1.5 mm, the coordinates (x, y) of the outer circumference of balance weight  122  are determined specifically as (expression-3) and (expression-4). 
 
 x=[ 10.0×cos (360°−θ)+37.3×cos {(sin −1 (10.0×sin (360−θ)/37.3)}+9.9−1.5]×cos (360°−θ)  (expression-3) 
 
 y=[ 10.0×cos (360°−θ)+37.3×cos {(sin −1 (10.0×sin (360°−θ)/37.3)}+9.9−1.5]×sin (360°−θ)  (expression-4) 
 
      In this configuration, in the closely approaching interval of balance weight  122  and piston  120 , the distance between outer circumference of balance weight  122  and piston  120  may be always kept constant at 1.5 mm. That is, in the structure having balance weight  122  disposed on a horizontal extension of piston  120 , in order to utilize effectively the space at the side of shaft  110  of piston  120 , by setting distance α at 2.0 mm or less, balance weight  122  having a large mass can be provided. Besides, by defining distance α at 1.5 mm, a sufficient design quality is obtained if considering fluctuations of dimension precision of parts.  
      The magnitude of inertial force obtained by rotation of balance weight  122  is proportional to the product of the distance from axial center  112   a  of eccentric shaft body  112  to the center of gravity of balance weight  122  and the mass of balance weight  122 . Therefore, according to the preferred embodiment, a greater inertial force can be applied as compared with balance weight  22  of nearly arc profile in the prior art. That is, the reciprocal inertial force of piston  120  can be canceled more effectively than in the prior art, and vibrations can be decreased without sacrificing the downsizing of compressor.  
      As the refrigerant, hitherto, tetrafluoroethane (R134a) has been generally used, but isobutane (R600a) is used in this preferred embodiment. The density of R600a is small, about 0.6 times that of R134a. Hence, in order to obtain the same refrigerating capacity as R134a, the required cylinder volume is about 1.7 times larger, and the mass of piston  120  is significantly increased. However, the embodiment incorporates balance weight  122  having a large inertial force in a limited space, and the reciprocal inertial force of piston  120  can be sufficiently canceled, and vibrations of the compressor can be decreased.  
      Besides, to realize high efficiency, the bearing is supported at two sides, and the overall height tends to be higher as compared with the bearing supported at one side. However, in the structure of disposing balance weight  122  on a horizontal extension of piston  120 , balance weight  122  having a large inertial force in a limited space can be provided. As a result, the overall height is not so much increased. That is, without sacrificing the downsizing of the compressor, a compressor of high efficiency and low vibration can be presented.  
      When forming balance weight  122  separately from shaft  110 , by employing a process capable of obtaining a dimensional precision close to the die precision such as sinter molding and iron plate presswork, a balance weight of a high dimensional precision can be obtained. As a result, distance a between the outer circumference of balance weight  122  and piston  120  can be shortened. That is, since balance weight  122  having a large inertial force in a limited space can be provided, vibrations of the compressor can be further decreased.  
      In the case of the bearing supported at both sides, by fixing balance weight  122  formed separately beneath subsidiary shaft body  133  by using bolts or rivets, assembling is easier, and the manufacturing cost of compressor can be lowered.  
      In the preferred embodiment, cylinder block  116  and main bearing  118  supporting main shaft body  111  are fixed by screws  123 , but main bearing  118  may be formed integrally in cylinder block  116 . In this case, same effects are obtained.  
      The smaller end side of connecting means  121  connecting with piston  120  has an annular shape, but a spherical ball joint may be also used. In this case, same effects are obtained.  
      Nearly same effects as in the invention are obtained by forming notch or dent in part of the outer circumference of balance weight  122 .  
      Electric moter element  105  is driven by inverter at plural operating frequencies including at least a frequency of 30 Hz or less that is an operating frequency of less than power source frequency, by a driving circuit (not shown). As a result, an appropriate refrigerating capacity can be obtained in a refrigerating machine for household use largely fluctuating in load such as refrigerator and freezer.  
      On the other hand, since electric moter element  105  and compression element  106  are elastically supported by way of suspension spring  107 , they have low eigenvalues. By inverter driving at operating frequency of lower than 30 Hz, vibrations of compression element  106  are close to the eigenvalues, and vibrations are increased by resonance. According to the preferred embodiment, since vibrations of compression element  106  can be decreased by applying a large inertial force by balance weight  122 , operation at low operating frequency of lower than 30 Hz can be realized.  
     INDUSTRIAL APPLICABILITY  
      As described herein, according to the invention, since the reciprocal inertial force of the piston can be canceled sufficiently by providing a balance weight having a sufficient inertial force in a limited space, on a horizontal extension of the piston, vibrations of hermetic compressor can be decreased. It hence presents a hermetic compressor of low vibration type which can be connected to a refrigeration cycle of refrigerator, air conditioner or freezer.