Abstract:
An expander includes a piston formed from a top portion, a skirt portion, and a land portion. A hollow heat-insulating space is formed within the piston making heat transfer difficult. It is possible to maintain the land portion, which is in contact with the high temperature, high pressure steam in an expansion chamber, at a high temperature, thereby minimizing any decrease in the temperature of the high temperature, high pressure steam to prevent any decrease in the efficiency of the expander. Any increase in the temperature of the skirt portion, which is in sliding contact with a cylinder sleeve, is suppressed to ensure the performance of the lubrication. Piston rings are provided on the land portion to separate the high temperature, high pressure steam in the expansion chamber from the oil in the skirt portion for preventing the oil and the high temperature, high pressure steam from being mixed together.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2003-392757 filed on Nov. 21, 2003 the entire contents of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an expander that includes a casing, a rotor rotatably supported in the casing, and an axial piston cylinder group arranged in the rotor so as to surround an axis of the rotor. The rotor is rotated by supplying high-temperature, high-pressure steam to an expansion chamber defined between a piston and a cylinder sleeve of the axial piston cylinder group. Sliding sections of the piston and the cylinder sleeve are lubricated with oil.  
         [0004]     2. Description of Related Art  
         [0005]     An expander is known as disclosed in Japanese Patent Application Laid-open No. 2002-256805. This expander includes a first axial piston cylinder group arranged on the radially inner side and a second axial piston cylinder group arranged on the radially outer side. A piston of the first axial piston cylinder group has a solid structure with one end facing an expansion chamber, to which high temperature, high pressure steam is supplied. The other end abuts against a swash plate, while sliding sections of the piston and a cylinder sleeve are lubricated with oil.  
         [0006]     In a piston of an axial piston cylinder group of an expander, it is desirable that an end thereof facing an expansion chamber is maintained at a high temperature so that the high temperature, high pressure steam supplied to the expansion chamber does not decrease. However, it is desirable for sliding sections of the piston and a cylinder sleeve to be maintained at a low temperature so as to ensure proper lubrication. However, if the piston has a solid structure, heat is quickly transmitted from the high temperature side to the low temperature side, so that the temperature of the end on the expansion chamber side, which should be maintained at a high temperature, easily decreases. In addition, the temperature of the sliding sections of the piston and the cylinder sleeve, which should be maintained at a low temperature, easily increases.  
       SUMMARY OF THE INVENTION  
       [0007]     The present invention has been developed to eliminate the above-mentioned circumstances. It is an object of the present invention to suppress the escape, via a piston, of the heat from the high temperature, high pressure steam supplied to an expansion chamber of an axial piston cylinder group of an expander, for ensuring that the lubrication performance is in sliding sections of the piston and a cylinder sleeve.  
         [0008]     In order to achieve the above-mentioned object, in accordance with a first aspect of the present invention, there is proposed an expander that includes a casing, a rotor rotatably supported in the casing, and an axial piston cylinder group arranged in the rotor so as to surround an axis of the rotor. The rotor is rotated by supplying high-temperature, high-pressure steam to an expansion chamber defined between a piston and a cylinder sleeve of the axial piston cylinder group, with sliding sections of the piston and the cylinder sleeve being lubricated with oil. The piston of the axial piston cylinder group has a land portion that is exposed to the high temperature, high pressure steam in the expansion chamber with a top portion that abuts against a swash plate, and a skirt portion that is disposed between the land portion and the top portion and slides against the cylinder sleeve. The piston has a hollow heat-insulating space formed therewithin. The land portion has a piston ring provided thereon with the piston ring separating the high temperature, high pressure steam in the expansion chamber from oil in the skirt portion.  
         [0009]     Furthermore, in accordance with a second aspect of the present invention, a plurality of piston rings are provided along the longitudinal direction of the piston.  
         [0010]     Moreover, in accordance with a third aspect of the present invention, a depression is provided between the plurality of piston rings.  
         [0011]     Furthermore, in accordance with a fourth aspect of the present invention, an oil channel is provided in at least one of an outer peripheral face of the skirt portion of the piston and an inner peripheral face of the cylinder sleeve.  
         [0012]     A second land channel  63   e  of an embodiment corresponds to the depression of the present invention, and a top ring  65  and a second ring  66  of the embodiment correspond to the piston rings of the present invention.  
         [0013]     In accordance with the arrangement of the first aspect, the piston of the axial piston cylinder group is formed from the land portion exposed to the high temperature, high pressure steam in the expansion chamber with the top portion abutting against the swash plate, and the skirt portion disposed between the land portion and the top portion and sliding against the cylinder sleeve. A hollow heat-insulating space is formed therewithin to make heat transfer difficult. Therefore, it is possible to maintain the land portion, which is in contact with the high temperature, high pressure steam supplied to the expansion chamber, at a high temperature, thereby minimizing any decrease in the temperature of the high temperature, high pressure steam to prevent any decrease in efficiency of the expander. Also, it is possible to suppress any increase in the temperature of the skirt portion, which is in sliding contact with the cylinder sleeve, to ensure lubrication performance. Moreover, since the piston ring provided on the land portion separates the high temperature, high pressure steam in the expansion chamber from the oil in the skirt portion, it is possible to prevent the oil from entering the expansion chamber side and cooling the land portion, and to prevent the high temperature, high pressure steam from entering the skirt portion side and degrading the lubricating effect of the oil.  
         [0014]     In accordance with the arrangement of the second aspect, since the plurality of piston rings are provided along the longitudinal direction of the piston, not only is it possible and more reliably to prevent the high temperature, high pressure steam from blowing past the land portion to the skirt portion, it is also possible to effectively prevent the oil and the high temperature, high pressure steam from mixing together by imparting the function of an oil ring to the piston ring which is on the side far from the expansion chamber.  
         [0015]     In accordance with the arrangement of the third aspect, the depression is provided between the plurality of piston rings. Therefore, even if some of the high temperature, high pressure steam blows past the piston ring on the expansion chamber side, it is possible to prevent, by the effect of the volume of the depression, a pressure difference between the expansion chamber side and the depression side of the piston ring from abruptly decreasing, to thus urge radially outwardly the piston ring by the pressure difference to prevent the piston ring from floating above an inner peripheral face of the cylinder sleeve or a piston ring channel face, thereby ensuring sealing performance.  
         [0016]     In accordance with the arrangement of the fourth aspect, since the oil channel is provided in at least one of the outer peripheral face of the skirt portion of the piston and the inner peripheral face of the cylinder sleeve, it is possible to retain oil in this oil channel to enhance the lubrication performance between the sliding surfaces of the piston and the cylinder sleeve.  
         [0017]     A mode for carrying out the present invention is explained below with reference to an embodiment of the present invention shown in the attached drawings.  
         [0018]     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred 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 this detailed description. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only, and thus are not limitative of the present invention, and wherein:  
         [0020]      FIG. 1  is a vertical sectional view of an expander;  
         [0021]      FIG. 2  is a sectional view along line  2 - 2  in  FIG. 1 ;  
         [0022]      FIG. 3  is a view taken along line  3 - 3  in  FIG. 1 ;  
         [0023]      FIG. 4  is an enlarged view of part  4  in  FIG. 1 ;  
         [0024]      FIG. 5  is an exploded perspective view of a rotor;  
         [0025]      FIG. 6  is a sectional view along line  6 - 6  in  FIG. 4 ;  
         [0026]      FIG. 7  is a sectional view along line  7 - 7  in  FIG. 4 ; and  
         [0027]      FIG. 8  is an enlarged view of part  8  in  FIG. 4 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0028]     An expander E of this embodiment is used in, for example, a Rankine cycle system. The expander E converts the thermal energy and the pressure energy of the high-temperature, high-pressure steam as a working medium into mechanical energy, and outputs the energy. A casing  11  of the expander E is formed from a casing main body  12 , a front cover  15  joined via a seal  13  to a front opening of the casing main body  12  by a plurality of bolts  14 , a rear cover  18  joined via a seal  16  to a rear opening of the casing main body  12  by a plurality of bolts  17 , and an oil pan  21  joined via a seal  19  to a lower opening of the casing main body  12  by a plurality of bolts  20 .  
         [0029]     A rotor  22  is arranged rotatably around an axis L extending in the fore-and-aft direction through the center of the casing  11 . The rotor  22  includes a front part supported by combined angular bearings  23  provided in the front cover  15 , and a rear part thereof supported by a radial bearing  24  provided in the casing main body  12 . A swash plate holder  28  is formed integrally with a rear face of the front cover  15 . A swash plate  31  is rotatably supported by the swash plate holder  28  via an angular bearing  30 . The axis of the swash plate  31  is inclined relative to the axis L of the rotor  22 , and the angle of inclination is fixed.  
         [0030]     The rotor  22  includes an output shaft  32  supported in the front cover  15  by the combined angular bearings  23  with three sleeve support flanges  33 ,  34 , and  35  being formed integrally with a rear part of the output shaft  32  via cutouts  57  and  58  having predetermined widths (see  FIG. 4  and  FIG. 8 ). A rotor head  38  is joined by a plurality of bolts  37  to the rear sleeve support flange  35  via a metal gasket  36  and is supported in the casing main body  12  by the radial bearing  24 . A heat-insulating cover  40  is fitted over the three sleeve support flanges  33 ,  34 , and  35  from the front and joined to the front sleeve support flange  33  by a plurality of bolts  39 .  
         [0031]     Sets of five sleeve support holes  33   a ,  34   a , and  35   a  are formed in the three sleeve support flanges  33 ,  34 , and  35  respectively at intervals of 72° around the axis L. Five cylinder sleeves  41  are fitted into the sleeve support holes  33   a ,  34   a , and  35   a  from the rear. A flange  41   a  is formed on the rear end of each of the cylinder sleeves  41 . An axial positioning is carried out by abutting the flange  41   a  against the metal gasket  36  while fitting the flange  41   a  into a step  35   b  formed in the sleeve support holes  35   a  of the rear sleeve support flange  35  (see  FIG. 8 ). A piston  42  is slidably fitted within each of the cylinder sleeves  41  with the front end of the piston  42  abutting against a dimple  31  a formed on the swash plate  31 . A steam expansion chamber  43  is defined between the rear end of the piston  42  and the rotor head  38 .  
         [0032]     A plate-shaped bearing holder  92  is superimposed on a front face of the front cover  15  via a seal  91  and is fixed thereto by means of bolts  93 . A pump body  95  is superimposed on a front face of the bearing holder  92  via a seal  94  and is fixed thereto by means of bolts  96 . The combined angular bearings  23  are held between a step of the front cover  15  and the bearing holder  92 , thereby fixing them in the axis L direction.  
         [0033]     A shim  97  having a predetermined thickness is held between the inner race of the combined angular bearings  23  and a flange  32   d  formed on the output shaft  32  supporting the combined angular bearings  23 . The inner race of the combined angular bearings  23  is tightened by a nut  98  screwed around the outer periphery of the output shaft  32 . As a result, the output shaft  32  is positioned in the axis L direction relative to the combined angular bearings  23 , that is, relative to the casing  11 .  
         [0034]     An oil passage  32   a  is formed so as to extend along the axis L within the output shaft  32  which is integral with the rotor  22 . The front end of the oil passage  32   a  branches in a radial direction and communicates with an annular channel  32   b  on the outer periphery of the output shaft  32 . An oil passage blocking member  45  is screwed into the inner periphery of the oil passage  32   a  via a seal  44  at a position that is radially inside the middle sleeve support flange  34  of the rotor  22 . A plurality of oil holes  32   c  extend radially outwardly from the oil passage  32   a  in the vicinity of the oil passage blocking member  45  and open on the outer periphery of the output shaft  32 .  
         [0035]     A trochoidal oil pump  49  is disposed between a recess  95   a  provided in a front face of the pump body  95  and a pump cover  48  fixed via a seal  46  to the front face of the pump body  95  by a plurality of bolts  47 . The trochoidal oil pump  49  includes an outer rotor  50  that is rotatably fitted in the recess  95   a , and an inner rotor  51  that is fixed to the outer periphery of the output shaft  32  and meshes with the outer rotor  50 . An internal space of the oil pan  21  communicates with an intake port  53  of the oil pump  49  via an oil pipe  52  and an oil passage  95   b  of the pump body  95 . A discharge port  54  of the oil pump  49  communicates with the annular channel  32   b  of the output shaft  32  via an oil passage  95   c  of the pump body  95 .  
         [0036]     The structure of the piston  42  is now explained in detail with reference to  FIG. 5  to  FIG. 8 .  
         [0037]     The piston  42  is formed from a top portion  61 , a skirt portion  62 , and a land portion  63 . The top portion  61  is a member having a spherical portion  61   a  that abuts against a dimple  31   a  of the swash plate  31 , and is joined to the front end of the skirt portion  62  by welding. The land portion  63  and the skirt portion  62  are formed integrally and have a large volume with an evacuated, heat-insulating space  64  defined therewithin. An annular oil channel  62   a  is formed by slightly decreasing the diameter in a middle section of the skirt portion  62  which is slidably fitted into an inner peripheral face of the cylinder sleeve  41 . A plurality of spiral channels  62   b  are formed in an outer peripheral section of the skirt portion  62  on the front side relative to the oil channel  62   a.    
         [0038]     The land portion  63  has a top land  63   a  on the expansion chamber  43  side and a second land  63   b  on the skirt portion  62  side. A top ring channel  63   c  is formed between the top land  63   a  and the second land  63   b  with a top ring  65  being fitted around the top ring channel  63   c . A second ring channel  63   d  is formed between the second land  63   b  and the skirt portion  62  with a second ring  66  being fitted around the second ring channel  63   d . A second land channel  63   e  is formed in a middle section of the second land  63   b . The top land  63   a  and the second land  63   b  have an outer diameter that is slightly smaller than the outer diameter of the skirt portion  62 . A gap a is formed between outer peripheral faces of the top land  63   a  and the second land  63   b  and the inner peripheral face of the cylinder sleeve  41 . Therefore, a thrust load in the peripheral direction for rotating the rotor  22  is transmitted from the skirt portion  62  to the cylinder sleeve  41  without passing through the land portion  63 .  
         [0039]     A top ring  65 , which is disposed at a position comparatively close to the end of the piston  42  (on the order of 10% of the diameter of the piston  42 ), has a rectangular cross-section with a face that is in sliding contact with the cylinder sleeve  41  which is a curved barrel face. A hard coating of a ceramic such as TiN or CrN is formed on the surface thereof. The second ring  66  has a rectangular cross-section (or an internal bevel cut cross-section); a face that is in sliding contact with the cylinder sleeve  41  which is a curved barrel face. A hard coating of a ceramic such as TiN or CrN is formed on the surface thereof. For both the top ring  65  and the second ring  66 , the gap across the ends is set to be the minimum gap at which the ends do not make contact when hot with the thickness being set as small as possible (on the order of 3% of the diameter of the piston  42 ), and the initial tension being set fairly small.  
         [0040]     Applying such a hard coating to the top ring  65  and the second ring  66  decreases the amount of wear and the amount of leakage of steam to be reduced. Since the initial tension of the top ring  65  and the second ring  66  is set to be small and the thickness is set to be small so that the load pushing the inner peripheral face of the cylinder sleeve  41  due to the pressure of the steam is reduced, it is possible to reduce the frictional force between the cylinder sleeve  41  and the top ring  65  and the second ring  66 , while enhancing the sealing effect by improving the ability of the top ring  65  and the second ring  66  to follow the cylinder sleeve  41 .  
         [0041]     An annular channel  41   b  is formed on the outer periphery of a middle part of the cylinder sleeve  41  (see  FIG. 5  and  FIG. 8 ), and a plurality of oil holes  41   c  are formed in the annular channel  41   b . The oil channel  62   a  formed in the middle section of the skirt portion  62  communicates with the oil holes  41   c  of the cylinder sleeve  41 .  
         [0042]     An annular cover member  69  is welded to the front side or the expansion chamber  43  side of the rotor head  38  which is joined to the rear face of the rear sleeve support flange  35  of the rotor  22  by the bolts  37 . Thus, an annular heat-insulating space  70  is defined at the back face or rear face of the cover member  69  (see  FIG. 8 ). The rotor head  38  is positioned rotationally relative to the rear sleeve support flange  35  by a knock pin  55 .  
         [0043]     As shown in  FIG. 1 , a rotary valve  71  is provided between the rear cover  18  of the casing  11  and the rotor head  38  of the rotor  22 . The rotary valve  71  supplies the high temperature, high pressure steam from a steam supply pipe  67  sequentially to five expansion chambers  43 , accompanying rotation of the rotor  22 , and discharges a low temperature, low pressure steam from the expansion chambers  43  to a steam discharge chamber  68  formed between the main body casing  12  and the rear cover  18 .  
         [0044]     The five cylinder sleeves  41  and the five pistons  42  form an axial piston cylinder group A of the present invention.  
         [0045]     The operation of the expander E of this embodiment having the above-mentioned arrangement is now explained.  
         [0046]     When the high temperature, high pressure steam generated by heating water in an evaporator that is supplied from the steam supply pipe  85  via the rotary valve  71  to the expansion chamber  43  within the cylinder sleeve  41 , the piston  42  fitted in the cylinder sleeve  41  is pushed forward from a top dead center toward a bottom dead center, so that the top portion  61  at the front end of the piston  42  pushes against the dimple  31  a of the swash plate  31 . As a result, the reaction force that the pistons  42  receive from the swash plate  31  gives a rotational torque to the rotor  22 . For each one fifth of a revolution of the rotor  22 , the high-temperature, high-pressure steam is supplied into a fresh adjoining expansion chamber  43 , thus continuously rotating the rotor  22 . While the piston  42 , having reached the bottom dead center accompanying the rotation of the rotor  22 , retreats toward the top dead center by being pushed by the swash plate  31 , the low-temperature, low-pressure steam pushed out of the expansion chamber  43  is discharged into the steam discharge chamber  68  via the rotary valve  71 .  
         [0047]     The oil pump  49  provided on the output shaft  32  operates together with the rotation of the rotor  22 . Oil is taken in from the oil pan  21  via the oil pipe  52 , the oil passage  95   b  of the pump body  95 , and the intake port  53  and is discharged from the discharge port  54 , and supplied to the oil channel  62   a  formed in the skirt portion  62  of the piston  42  via the oil passage  95   c  of the pump body  95 , the oil passage  32   a  of the output shaft  32 , the annular channel  32   b  of the output shaft  32 , the oil holes  32   c  of the output shaft  32 , the annular channel  41   b  of the cylinder sleeve  41 , and the oil holes  41   c  of the cylinder sleeve  41 . A portion of the oil retained by the oil channel  62   b  flows into the spiral oil channels  62   b  formed in the skirt portion  62  of the piston  42 , lubricates the surface that slides against the cylinder sleeve  41 , and is then returned to the oil pan  21 . Another portion of the oil lubricates the surfaces of the top ring  65  and the second ring  66  that slide against the cylinder sleeve  41 , the top ring  65  and the second ring  66  are provided in the land portion  63  of the piston  42 . Since the oil channel  62   a  formed in the skirt portion  62  has the function of temporarily retaining oil, it is possible to continuously supply oil to the sliding sections of the piston  42  and the cylinder sleeve  41 , thus improving the lubrication conditions.  
         [0048]     Since the evacuated, heat-insulating space  64  is formed in the interior of the piston  42 , it is possible to suppress the escape, via the piston  42 , of the heat of high temperature, high pressure steam supplied to the expansion chamber  43  which faces the end of the piston  42 , thus minimizing any decrease in the temperature of the high temperature, high pressure steam in the expansion chamber  43  to increase the output of the expander E. Furthermore, since the top of the piston  42  is maintained at a high temperature, it becomes difficult for the steam to condense and liquify between the land portion  63  of the piston  42  and the cylinder sleeve  41 , thus improving the lubrication conditions of the land portion  63  to improve the sealing performance and the wear resistance of the top ring  65  and the second ring  66 .  
         [0049]     Since the rear side of the top ring channel  63   c  of the land portion  63  of the piston  42  communicates with the expansion chamber  43  which is at a high pressure, and the front side thereof communicates with the second land channel  63   e  which is at a low pressure, the difference in pressure pushes the top ring  65  from the bottom part of the top ring channel  63   c  to make the top ring  65  come into intimate contact with the inner peripheral face of the cylinder sleeve  41  and the side face of the top ring channel  63   c , thereby improving the sealing performance. Even if a part of the high temperature, high pressure steam in the expansion chamber  43  blows past the top ring  65  toward the second land channel  63   e , it is possible to suppress an abrupt decrease in the difference in pressure by virtue of the volume of the second land channel  63   e , thus maintaining the tension of the top ring  65  to prevent the sealing performance from deteriorating. Moreover, the second land channel  63   e  also has the function of retaining oil, thus contributing to an improvement in the lubrication performance.  
         [0050]     The second ring  66  maintains the compression when the high temperature, high pressure steam blows past the top ring  65 , and has the function of an oil ring to scrape off oil attached to the inner peripheral face of the cylinder sleeve  41 . In this way, the high temperature, high pressure steam in the expansion chamber  43  is separated from the oil in the skirt portion  62  by means of the top ring  65  and the second ring  66  provided on the land portion  63 , to thereby prevent the oil from entering the expansion chamber  43  side and cooling the land portion  63 . Thus, the high temperature, high pressure steam is prevented from entering the skirt portion  62  side and degrading the lubricating effect of the oil.  
         [0051]     Although an embodiment of the present invention is explained above, the present invention can be modified in a variety of ways without departing from the subject matter thereof.  
         [0052]     For example, the axial piston cylinder group A of the embodiment includes the five pistons  42  and the five cylinder sleeves  41 , but the numbers thereof are not limited to those of the embodiment.  
         [0053]     Furthermore, the heat-insulating space  64  of the embodiment is evacuated. However, the heat-insulting space  64  may have a gas such as air sealed inside.  
         [0054]     Moreover, in the embodiment, the oil channel  62   a  is provided in the outer peripheral face of the piston  42 , but an oil channel may be provided in the inner peripheral face of the cylinder sleeve  41 .  
         [0055]     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.