Abstract:
An insertion hole ( 83   a ) is provided in a packing ( 83 ) and a rotating shaft ( 81   a ) is inserted into the insertion hole ( 83   a ). A sliding portion ( 83   b ) forming an air clearance ( 83   c ) is provided on the outside of the packing ( 83 ). The rotating shaft ( 81   a ) is inserted into the insertion hole ( 83   a ) with a grease G applied to the circumferential surface thereof. When the rotating shaft ( 81   a ) is inserted, the grease G is shaved off by the sliding portion ( 83   b ) and a sufficient amount of the grease G is stored in the air clearance ( 83   c ). Consequently, frictional sound and heat are and thus reduced between the rotating shaft ( 81   a ) and the packing ( 83 ), the sliding portion ( 83   b ) can come into tight contact with the rotating shaft ( 81   a ), thereby enhancing airtightness.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a steam cooking device that jets steam into a heating chamber to cook an article-to-be-heated. 
     2. Description of the Related Art 
     A conventional steam cooking device is disclosed in Patent Document 1. This conventional steam cooking device uses superheated steam as a heating medium, and an article-to-be-heated is placed on a tray disposed in a heating chamber. It is also provided with a blower fan for circulating gas inside the heating chamber. A sectional view of a conventional structure of a blower fan for steam cooking devices is shown in the sectional view of  FIG. 6 . As shown in  FIG. 6 , a blower fan  100  is provided with a motor  101  and an impeller  102 . The impeller  102  is disposed inside a partition wall  110  that isolates gas present in the heating chamber from the outside. The motor  101  is disposed outside the partition wall  110 . In the partition wall  110 , a through hole  110   a  is formed, into which a packing  103  is fitted. In the packing  103 , an insertion hole  103   a  is formed, into which a rotation shaft  101   a  of the motor  101  is inserted.
     Patent Document 1: JP-A-2005-351510   

     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, according to the above-described conventional steam cooker, at openings at two ends of the packing  103 , shaft supporting portions  103   b  and  103   c  are formed to protrude toward the inside of the openings. With this structure, when the rotation shaft  101   a  with grease applied to the peripheral surface thereof is inserted into the through hole  103   a  of the packing  103 , the grease is scraped off by the shaft supporting portions  103   b  and  103   c . As a result, the amount of grease stored inside the packing  103  is reduced, and this makes noise and heat liable to be caused by friction between the rotation shaft  101   a  and the packing  103 . On the other hand, if the closeness of contact between the rotation shaft  101   a  and the packing  103  is reduce reduced by providing a space therebetween for the purpose of reducing friction noise and friction heat, air outside the partition wall  110  is more likely to flow into the inside of the partition wall  110 . This deteriorates airtightness, to disadvantageously increase the oxygen concentration inside the partition wall  110  and inside the heating chamber. If the airtightness is not sufficient here, it is difficult to maintain the heating chamber in a super low-oxygen state in which the oxygen concentration is on the order of 0.1%. This results in an inconvenience that components that are easily-oxidizable in high-temperature atmosphere such as vitamin C are oxidized while food is being cooked. 
     In view of the above inconveniences, an object of the present invention is to provide a steam cooking device in which airtightness is improved while reducing friction between a rotation shaft and packing, and thereby a super low-oxygen state in which the oxygen concentration is on the order of 0.1% can be maintained, and a method for manufacturing such a steam cooking device. Another object of the present invention is to provide packing capable of improving closeness of contact thereof with respect to a rotation shaft that is inserted therethrough while reducing friction therebetween. 
     Means for Solving the Problem 
     To achieve the above object, according to one aspect of the present invention, a packing for holding a shaft that, in a state in which a rotation shaft is inserted into an insertion hole formed in the packing, rotatably holds the rotation shaft has the following features. That is, an inside of the insertion hole which faces the rotation shaft comprises a sliding contact portion having a surface that is in sliding contact with the rotation shaft and a surface that is not in contact with the rotation shaft, the surface that is not in contact with the rotation shaft forms a clearance an end of which in an axial direction is open, and part of the clearance extends over an outer surface of the sliding contact portion. 
     According to this structure, the sliding contact portion with which the rotation shaft is in sliding contact is formed in the insertion hole. The clearance one end of which in the axial direction is open is provided on the side of the outer surface of the sliding contact portion, and the sliding contact portion is supported at an end at which the clearance is not open. Grease is applied to a circumferential surface of the rotation shaft, and the rotation shaft is then inserted through the insertion hole from the side on which the clearance is open, to be in sliding contact with the sliding contact portion. Part of the grease is scraped off at an end face of the sliding contact portion to be stored in the clearance. Furthermore, a width of the sliding contact portion in an axial direction is smaller than a width of the insertion hole in an axial direction, and an inner diameter of the end face of the clearance at which the clearance is open is larger than an inner diameter of the sliding contact portion. As a result, the rotation shaft having the grease applied to its circumferential surface is inserted into the end face of the clearance at which the clearance is open, with a space therebetween, to slidingly contact the sliding contact portion. 
     According to the present invention, it is preferable that, in the above-structured packing, the clearance be narrow at an end face where the clearance is open. 
     According to the present invention, it is preferable that, in the above-structured packing, an outer surface of the clearance be an inclined surface that extends to an end face of the insertion hole. With this structure, the outer surface of the clearance is a smooth continuous surface. 
     According to the present invention, it is preferable that, in the above-structured packing, the outer surface of the sliding contact portion be formed as a surface inclined such that a diameter of the sliding contact portion is smaller closer to the end of the clearance at which the clearance is open. 
     According to the present invention, it is preferable that, in the above-structured packing, two ends of an outer surface of the packing have different shapes. 
     According to another aspect of the present invention, a steam cooking device is provided with the packing according to claim  1 , a heating chamber having airtightness, a steam generating device that generates steam, and jets the steam into the heating chamber, a motor provided with the rotation shaft, a blower fan having an impeller rotated by the motor to circulate gas in the heating chamber, and a partition wall that has a through hole that the rotation shaft penetrates, that is located between the motor and the impeller, and that isolates the motor from circulating gas. Here, the through hole is sealed with the packing through which the rotation shaft is inserted. 
     According to this structure, steam generated by the steam generating device is jetted into the heating chamber. The motor of the blower fan is disposed outside the partition wall, and the impeller is disposed in a circulation passage. When the blower fan is driven, the impeller rotates to circulate the steam present in the heating chamber, whereby an article-to-be-heated is cooked. The packing having the insertion hole is fitted to the through hole formed in the partition wall, and the rotation shaft of the motor is inserted through the insertion hole to seal the through hole. 
     According to the present invention, it is preferable that, in the above-structured steam cooking device, the packing be disposed such that the end of the clearance at which the clearance is open is located on the motor side. 
     According to still another aspect of the present invention, a method for manufacturing a steam cooking device, which includes the packing according to claim  1 , an airtight heating chamber, a steam generating device that generates steam and jets the steam into the heating chamber, a motor provided with the rotation shaft, a blower fan having an impeller rotated by the motor to circulate gas in the heating chamber, and a partition wall that has a through hole that the rotation shaft penetrates, that is located between the motor and the impeller, and that isolates the motor from circulating gas, the through hole being sealed with the packing through which the rotation shaft is inserted, includes steps of: applying grease to a circumferential surface of the rotation shaft; and inserting the rotation shaft, to the circumferential surface of which the grease is applied, through the packing starting at an end of the packing on a side on which the clearance is open. 
     Advantages of the Invention 
     According to the present invention, since a sliding contact portion is formed in an insertion hole of a packing, and a clearance is provided outside the sliding contact portion, part of grease applied to the circumferential surface of a rotation shaft can be scraped off to be stored in the clearance. This allows a sufficient amount of grease to be stored inside the packing, and helps alleviate grease reduction that causes friction noise and friction heat. Thus, the packing can be structured without a space between the sliding contact portion and the rotation shaft, so that the sliding contact portion and the rotation shaft are in close contact with each other, and thus the airtightness of the packing can be improved. 
     Also, since part of the sliding contact portion is surrounded by the clearance in the packing, if runout of the rotation shaft occurs, the sliding contact portion easily changes its shape on receiving contact pressure from the rotation shaft, to thereby cope with the runout of the rotation shaft. As a result, the sliding contact portion can be kept in close contact with the rotation shaft without increasing contact pressure when runout of the rotation shaft has occurred. This helps prevent occurrence of a gap between the rotation shaft and the sliding contact portion to further improve the airtightness of the packing, and this also helps reduce friction between the rotation shaft and the sliding contact portion when an axial run-out of the rotation shaft has occurred. 
     Also, since the width of the sliding contact portion in the axial direction is formed smaller than the width of the insertion hole in the axial direction, generation of friction noise and friction heat can be reduced more effectively. Moreover, since the inner diameter of the end face of the insertion hole on the side on which the clearance is open is larger than the inner diameter of the sliding contact portion, grease is less prone to stick to an end face of the packing, and thus a process of wiping grease off the packing can be omitted. 
     Also, according to the present invention, the end face of the clearance on the side on which the clearance is open is narrow. This helps prevent leakage of grease inserted into the clearance, and also helps reduce intrusion of foreign substances into the insertion hole. 
     Also, according to the present invention, since the outer surface of the clearance is an inclined surface extending to the end face of the insertion hole, the packing can be smoothly demolded in its manufacturing process. This facilitates the manufacturing of the packing. 
     Also, according to the present invention, the outer surface of the sliding contact portion is formed as a surface inclined such that the outer diameter of the sliding contact portion is smaller closer to the side on which the clearance is open. As a result, grease is easily led to the outside of the sliding contact portion, and this facilitates storage of the grease. 
     Also, according to the present invention, since the two ends of the outer surface of the packing have different shapes, the side on which the clearance is open can be visually recognized. This makes it easy to decide the direction in which the packing should be fitted, which helps prevent erroneous fitting of the packing. 
     Also, according to the present invention, a steam cooking device is provided with a highly airtight packing. This makes it possible to maintain the heating chamber in a super low-oxygen state where the oxygen concentration is on the order of 0.1% during a cooking operation. As a result, oxidation of an article-to-be-heated can be prevented, and thus, destruction of nutrients such as vitamin C can be prevented. 
     Also, according to the present invention, since the side of the packing on which the clearance is open is located on the side of the motor, the grease stored in the clearance can be prevented from coming in contact with steam. This helps prevent deterioration of the grease due to steam. 
     Also, according to the present invention, the rotation shaft is inserted through the packing after grease is applied to the circumferential surface of the rotation shaft. In this way, grease can be led to the inside of the packing more easily than in a case in which grease is applied to the inside of the packing. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  A perspective view showing a steam cooking device according to an embodiment of the present invention; 
         FIG. 2  A perspective view showing a steam cooking device according to an embodiment of the present invention with a door open; 
         FIG. 3  A diagram schematically showing an inside structure of a steam cooking device according to an embodiment of the present invention; 
         FIG. 4  A sectional view showing a blower fan in a steam cooking device according to an embodiment of the present invention; 
         FIG. 5A  A sectional view showing packing and a rotation shaft in a steam cooking device according to an embodiment of the present invention; 
         FIG. 5B  A sectional view showing packing and a rotation shaft in a steam cooking device according to an embodiment of the present invention; and 
         FIG. 6  A sectional view showing a blower fan in a conventional steam cooking device. 
     
    
    
     LIST OF REFERENCE SYMBOLS 
     
         
         
           
               1  steam cooking device 
               11  door 
               20  heating chamber 
               21  plate 
               27  partition wall 
               27   a  through hole 
               28  inlet port 
               31  exhaust fan 
               32 ,  33  exhaust duct 
               34  steam supply duct 
               35  circulation duct 
               40  steam heating device 
               41  steam heating heater 
               47  outer cover 
               48  damper 
               50  steam generating device 
               51  pot 
               52  steam generating heater 
               54  water drain valve 
               55  water supply passage 
               57  water supply pump 
               61  jet cover 
               65 ,  67  jet port 
               70  tank case 
               71  water tank 
               71   a  inlet port 
               80  blower fan 
               81  motor 
               81   a  rotation shaft 
               81   b  fixed portion 
               82  impeller 
               82   a  boss 
               82   b  blade 
               82   c  boss hole 
               82   d  screw 
               83  packing 
               83   a  insertion hole 
               83   b  sliding contact portion 
               83   c  clearance 
               83   d  first protruding portion 
               83   e  second protruding portion 
               83   f  end face 
               91  temporary reservoir 
               92  pot water-level detection portion 
             D rotation shaft diameter 
             d inner diameter of sliding contact portion 
             L length of sliding contact portion in axial-direction 
             F article-to-be-heated 
             G grease 
           
         
       
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, embodiments of the present invention will be described with reference to the drawings.  FIG. 1  is a perspective view showing a steam cooking device according to an embodiment of the present invention. The steam cooking device  1  shown in  FIG. 1  is for cooking an article-to-be-heated by using superheated steam. The steam cooking device  1  has a cabinet  10  in the shape of a rectangular parallelepiped, and a door  11  is provided on the front face of the cabinet  10 . 
     The door  11  is supported on the cabinet  10  so as to be pivotable about the bottom edge of the door  11  in a vertical plane, and a handle  12  is fitted in an upper part of the door  11 . A middle part  11 C of the door  11  has a pane of heat-resistant glass set therein to form a see-through portion  14 . On the left and right of the middle part  11 C, a left-side part  11 L and a right-side part  11 R, each finished with a metal decoration plate, are arranged symmetrically. On the right-side part  11 R, an operation panel  13  is provided. 
       FIG. 2  is a perspective view showing the steam cooking device  1 , with the door  11  open. As shown in  FIG. 2 , when the handle  12  provided in an upper part of the door  11  is held and pulled frontward, the door  11  changes its position through 90° from a vertical, closed state to a horizontal, open state. When the door  11  is open, the front face of the cabinet  10  appears. 
     A heating chamber  20  is provided in a position corresponding to the middle part  11 C of the door  11  in the front face of the cabinet  10  that appears when the door  11  is opened. The heating chamber  20  is formed substantially in the shape of a rectangular parallelepiped, and the front face thereof at which it faces the door  11  is formed as a completely open opening through which an article-to-be-heated F (see  FIG. 3 ) is put in or taken out from the heating chamber  20 . The door  11  is rotated to open/close the opening. Wall surfaces of the heating chamber  20  are formed of stainless steel plates, and heat insulation is applied to outer surfaces of the heating chamber  20 . The heating chamber  20  is airtight, and is tightly closed when the door  11  is closed. In the heating chamber  20 , a tray  21  is placed, and, above the tray  21 , a rack  22  formed of stainless steel wire is placed for placing an article-to-be-heated F (see  FIG. 3 ) thereon. 
     The front face of the cabinet  10  appears when the door  11  is opened, and a part of the front face corresponding to the right-side part  11 R of the door  11  is not formed as an opening. Inside this part of the cabinet  10 , a control circuit board (not shown) is disposed. On the other hand, inside a part of the front face of the cabinet  10  corresponding to the left-side part  11 L of the door  11 , a tank case  70  is disposed. In the tank case  70 , there is accommodated a water tank  71  in which water for generating steam and water discharged from a steam generating device  50  (see  FIG. 3 ) is stored. 
     Next, the inside structure of the steam cooking device  1  will be schematically described with reference to  FIG. 3 .  FIG. 3  is a diagram schematically showing the inside structure of a steam cooking device  1 . In the figure, the outline arrow indicates the direction in which steam flows. In the figure, the heating chamber is shown as seen from a side, and for ease of understanding the inside structure of the steam cooking device  1 , the water tank  71 , the steam generating device  50  and the like are illustrated outside the cabinet  10 . In practice, as shown in  FIG. 2 , the water tank  71  is disposed beside the heating chamber  20  inside the cabinet  10 , and the steam generating device  50  is also disposed inside the cabinet  10 . 
     As shown in  FIG. 3 , the water tank  71  communicates with a temporary reservoir  91  via a joint portion  58  provided in the tank case  70 . In this way, the water tank is detachable with respect to the cabinet  10  (see  FIG. 2 ). 
     A water supply passage  55  extends to the bottom of the temporary reservoir  91  to be immersed therein. Midway in the water supply passage  55 , a water supply pump  57  is provided to be connected to the steam generating device  50 . The steam generating device  50  has a cylindrical pot  51  whose axial direction is substantially equal to the vertical direction, and when the water supply pump  57  is driven, water is supplied from the water tank  71  to the pot  51 . 
     The pot  51  is formed of a metal, synthetic resin, or ceramic, or of a combination of these different materials, and has thermal resistance. Inside the pot  51 , a steam generating heater  52  formed with a spiral sheath heater is immersed therein. When the steam generating heater  52  is energized, the water present in the pot  51  is heated, and thereby steam is generated. 
     Inside the pot  51 , a cylindrical partition  51   a  is formed to extend from the top surface of the pot  51  into the spiral steam generating heater  52 . Inside the partition  51   a , a pot water-level detection portion  92  is provided for detecting the water level in the pot. The pot water-level detection portion  92  has a plurality of electrodes, and detects the water level in the pot  51  based on electrical conduction between the electrodes. 
     The partition  51   a  is provided for the purpose of making bubbles resulting from the boiling of water less likely to affect the pot water-level detection portion  92 . This improves the detection accuracy of the pot water-level detection portion  92 . 
     From the top surface of the pot  51 , a steam supply duct  34  extends to be connected to a circulation duct which will be described later. At an upper part of the peripheral surface of the pot  51 , an overflow water pipe  98  is provided to be connected to the temporary reservoir  91 . Through the overflow water pipe  98 , water overflowing from the pot  51  is led to the temporary reservoir  91 . The water level at which water overflows from the pot  51  into the overflow water pipe  98  is set higher than the ordinary water level in the pot  51  but lower than the steam supply duct  34 . 
     The pot  51  has a funnel-shaped bottom part, from a lower end of which a water drain pipe  53  extends. Midway along the water drain pipe  53 , a water drain valve  54  is provided. The water drain pipe  53  extends to an inlet port  71   a  of the water tank  71  with a slope of a predetermined angle. The inlet port  71   a  is provided with a mesh filter (not shown) for filtering out wastes from drained water. Water stored in the pot  51  can be discharged into the water tank  71  by opening the valve  54 , and can be discarded by detaching the water tank  71 . 
     A circulation duct  35  is provided from the rear surface to the top surface of the outer wall of the heating chamber  20 . Part of the rear wall of the heating chamber  20  is formed open as an inlet port  28  of the circulation duct  35 , and the circulation duct  35  is connected to the steam heating device  40  disposed at an upper portion of the heating chamber  20 . A lower surface of the steam heating device  40  is covered by a jet cover  61 , and an upper surface of the steam heating device  40  is covered by an upper cover  47 . 
     Both upper and lower surfaces of the jet cover  61  are finished to be dark-colored by surface treatment such as painting. As a result, the jet cover  61  can absorb heat radiated from a steam heating heater  41  well, and can radiate heat from the lower surface thereof into the heating chamber  20 . Furthermore, the jet cover  61  is formed to protrude to the inside of the heating chamber, has a plurality of jet ports  65  formed in the lower surface thereof, and has a plurality of jet ports  67  formed in a front surface thereof. Although not shown, jet ports are also formed in side surfaces of the jet cover  61 . 
     The steam heating device  40  is provided with the steam heating heater  41  formed with a sheath heater, and further heats steam generated by the steam generating device  50  to generate superheated steam. The steam heating device  40  is disposed, in plan view, in the middle of the ceiling portion of the heating chamber  20 . Furthermore, the steam heating device  40  is formed to be smaller in area than the top surface of the heating chamber  20  and small in volume to obtain high heating efficiency. 
     Inside the circulation duct  35 , there is disposed a blower fan  80  formed with a centrifugal fan, and the steam supply duct  34  is connected to an upstream side of the blower fan  80 . When the steam blower fan is driven, steam generated by the steam generating device  50  flows into the circulation duct  35  via the steam supply duct  34  to be fitted into the heating chamber  20  through the jet ports  65  and  67 . Part of the steam jetted into the heating chamber  20  circulates such that it is sucked through the inlet port  28  to flow through the circulation duct to be jetted out again through the jet ports  65  and  67  of the jet cover  61 . Detailed description of the structure of the blower fan  80  will be given later. 
     Furthermore, an exhaust duct  33  branches out from an upper portion of the circulation duct  35  via a damper  48 . The exhaust duct  33  has an open end that is open to the outside, and forcibly discharges gas in the heating chamber  20  when the damper  48  is opened and the blower fan  80  is driven. Moreover, at a lower portion of the heating chamber  20 , an exhaust port  32   a  is formed, and an exhaust duct  32  extends from the exhaust port  32   a . The exhaust duct  32  is formed of metal such as stainless steel, and has an open end open to the outside, through which the exhaust duct  32  naturally exhausts gas in the heating chamber out of the device. 
     In an ordinary state, that is, in a state in which steam cooking is not being performed, air is dominant inside the heating chamber  20 . On the other hand, when steam cooking is started, as described above, superheated steam is jetted into the heating chamber  20 , and air and steam is exhausted out of the heating chamber  20 . As a result, a steam atmosphere is achieved in the heating chamber  20 . Oxygen in the air has been sufficiently exhausted in this state, and this helps reduce oxidation of the article-to-be-heated F. 
     The steam cooking device  1  may also be provided with a magnetron so that cooking can also be performed by using a microwave. When cooking is performed by using a microwave, outside air is sucked in via the exhaust duct  32 . 
       FIG. 4  is a sectional view showing the blower fan  80  of this embodiment. The blower fan  80  is provided with a motor  81  and an impeller  82 . The impeller  82  is disposed inside the circulation duct  35 , and the motor  81  is disposed outside the circulation duct  35 . A partition wall  27  of the circulation duct  35  isolates the motor  81  from the circulating gas. A through hole  27   a  is formed in the partition wall  27 , and packing  83  is fitted to the through hole  27   a . The packing  83  is a flexible member formed of a fluororubber, a tetrafluoroethylene resin (PTFE), or the like by a method such as mold processing. In the packing  83 , an insertion hole  83   a  is formed, and a rotation shaft  81   a  of the motor  81  is inserted into the insertion hole  83   a.    
     The impeller  82  is composed of a boss  82   a  disposed at the center and a plurality of blades  82   b  arranged along a circumference of the boss  82   a . The boss  82   a  has a cylindrical shape, and the rotation shaft  81   a  is fitted into the boss  82   a . In the boss  82   a , a boss hole  82   c  is formed to penetrate through the boss  82   a . A screw  82   d  is screwed into the boss hole  82   c  to pressed the rotation shaft  81   a . In this way, the boss  82   a  and the rotation shaft  81   a  are fixed to each other. 
     A magnetic field is generated when power is supplied to the motor  81 , and the motor  81  rotates the rotation shaft  81   a  by magnetic force. The rotation of the rotation shaft  81   a  makes the impeller  82  rotate. The rotation of the impeller  82  makes gas flow in the circulation duct  35 , and as a result, pressure is lower inside the partition wall  27  than outside the partition wall  27 . In this state, the packing  83  prevents air from flowing to the inside of the partition wall from the outside thereof. 
       FIGS. 5A and 5B  are sectional views showing the packing  83  and the rotation shaft  81   a . As shown in  FIG. 5A , in the insertion hole  83   a  formed in the packing  83 , there is provided a sliding contact portion  83   b  having a surface with which the rotation shaft  81   a  is in sliding contact. Furthermore, a clearance  83   c  is formed with a surface that is not in contact with the rotation shaft  81   a . An end of the clearance  83   c  in an axial direction is open, and part of the clearance  83   c  extends over the outer surface of the sliding contact portion  83   b . And, the sliding contact portion  83   b  is supported in the packing  83  at another end of the clearance that is not open. 
     A width of the sliding contact portion  83   b  in an axial direction is formed smaller than a width of the insertion hole  83   a  in an axial direction. An end face  83   f  of the clearance  83   c  on the side at which the clearance  83   c  is open is narrow. A diameter of the end face  83   f  located on the side at which the clearance  83   c  is open is made wider than the outer diameter of the sliding contact portion  83   b.    
     At both ends of the packing  83 , first and second protruding portions  83   d  and  83   e  are respectively provided protruding outward in the diameter direction. Between the first and second protruding portions  83   d  and  83   e , the partition wall  27  (see  FIG. 4 ) is disposed, and thereby the packing  83  is fitted to the partition wall  27  (see  FIG. 4 ). An outer diameter of the second protruding portion  83   e  provided at an end portion of the packing  83  on which the clearance  83   c  is open is smaller than an outer diameter of the first protruding portion that is provided on the other end portion. This makes it easy to recognize the side at which the clearance  83   c  is open by merely looking at the packing  83 . As a result, the packing  83  can be fitted in a correct direction in the manufacturing process of the steam cooking device  1  (see  FIG. 4 ), and this helps reduce the number of steps in the manufacturing process. Note that the outer diameter of the second protruding portion  83   e  may be larger than that of the first protruding portion  83   d . Also, since the side on which the clearance  83   c  is open can be easily recognized as long as the two ends of the outer surface of the packing have different shapes, the outer surface of the packing may have any shape other than the above described shape protruding outward in the diameter direction. 
     At an end portion of the rotation shaft  81   a , a depressed fixing portion  81   b  is provided to thereby fix the rotation shaft  81   a  to the impeller  82 . The rotation shaft  81   a  is inserted into the packing  83  from the end thereof at which the fixing portion  81   b  is formed. At this time, the rotation shaft  81   a  is inserted into the end face  83   f  located on the side at which the clearance  83   c  is open. Grease G has been applied to the circumferential surface of the rotation shaft  81   a . The grease G is applied to the circumferential surface of the rotation shaft  81   a  particularly at a portion that comes in contact with the rotatable contact portion  83   b  when the rotation shaft  81   a  is fitted in position. 
     Assuming that a diameter D of the rotation shaft  81   a  is 4 mm, an inner diameter d of the sliding contact portion  83   b  is preferably on the order of 3.95 to 4.15 mm. A length L of the sliding contact portion  83   b  in an axial direction is preferably 2 to 3 mm, and more preferably, 3 mm. A thickness of the sliding contact portion  83   b  is preferably 0.3 to 0.7 mm, and more preferably, 0.3 mm. A thickness of the clearance  83   c  is preferably 0.5 to 0.8 mm. Hardness of the flexible material of which the packing  83  is formed is preferably 65 to 70 degrees of rubber hardness (JIS K 6253). 
     As shown in  FIG. 5B , when the rotation shaft  81   a  is inserted into the packing  83 , part of the grease G is scraped off by the sliding contact portion  83   b . The grease G that is scraped off at this time is stored in the clearance  83   c . As a result, a sufficient amount of grease G is stored in the insertion hole  83   a  of the packing  83 . Also, since the outer peripheral surface of the sliding contact portion  83   b  is formed as a surface inclined such that the outer diameter of the sliding contact portion is smaller closer to the side on which the clearance is open, grease G can be easily stored in the clearance  83   c.    
     Also, the clearance  83   c  is made narrow at the end face  83   f  located on the side at which the clearance  83   c  is open. This prevents the grease G from leaking from inside to outside of the packing  83 . Furthermore, this prevents mixing-in of foreign substances from outside the packing  83 . Here, in the packing  83 , the outer surface of the clearance  83   c  is formed as a smooth inclined surface extending to the end face of the insertion hole  83   a , and furthermore, the packing  83  is formed of a flexible material. This prevents the packing  83  from being stuck when it is taken out from the mold after being molded. This facilitates the manufacturing of the packing  83 . 
     Next, the operation of the steam cooking device  1  will be described referring to  FIGS. 1 to 4  and  FIGS. 5A and 5B  mentioned above. First, in the steam cooking device  1  having the above structure, the door  11  is opened and the water tank  71  is taken out from the tank case  70 . At this time, the joint portion  58  is disengaged and a water stop valve is closed. The water tank  71  is carried by a user to where the user fills it with, for example, tap water. 
     Then the user carries the water tank  71  back to the steam cooking device  1  to insert it into the tank case  70 . At this time, the water tank  71  is connected to the temporary reservoir  91  by the joint portion  58 , and the water stop valve is opened. 
     The user puts the article-to-be-heated F on the rack  22 , closes the door  11 , selects an option from a menu by operating an operation panel  13 , and presses down a start key (not shown). Thereby, a sequence of cooking is started, and the operation of the water supply pump  57  is started to supply water to the steam generating device  50 . At this time, the water drain valve  54  is in a closed state. 
     With the water supply pump  57  driven, water is supplied into the pot  51  through the water supply passage  55  to a predetermined water level, and then the water supply is stopped. With a predetermined amount of water in the pot  51 , the steam generating heater  52  is energized to directly heat the water present in the pot  51 . 
     The blower fan  80  and the steam heating heater  41  are energized when the steam generating heater  52  is energized, or when the temperature of the water in the pot  51  reaches a predetermined level. With the blower fan  80  driven, steam in the heating chamber  20  is sucked into the circulation duct  35  from the inlet port  28 . When the water present in the pot  51  boils, steam of 1 atmosphere pressure and of a temperature of 100° C. is generated, and saturated steam flows into the circulation duct  35  via the steam supply duct  34 . At this time, the damper  48  is in a closed state. The steam is forced by the blower fan  80  to flow through the circulation duct  35  and then into the steam heating device  40 . 
     The steam that has flown into the steam heating device  40  is heated by the steam heating heater  41  to become superheated steam having a temperature of 100° C. or higher. Typically used is superheated steam heated to 150 to 300° C. Part of the superheated steam is jetted in a straight downward direction (the direction indicated by arrow A) from the jet ports  65 . Thereby, a top surface of the article-to-be-heated F comes into contact with the superheated steam. 
     Part of the superheated steam is jetted sideways in an obliquely-downward direction from the jet ports formed in the side surfaces of the jet cover  61 . The superheated steam jetted sideways is reflected on a reflection portion (not shown) formed as a depression in a side wall of the heating chamber  20 , to be led below the article-to-be-heated F. Thereby, a lower surface of the article-to-be-heated F comes into contact with the superheated steam. 
     When the temperature of the surface of the article-to-be-heated F is 100° C. or lower, the superheated steam is condensed on the surface of the article-to-be-heated. At this time, a large amount of heat is applied in the form of condensation heat, which is as large as 539 cal/g, and thereby the article-to-be-heated F can be heated quickly. 
     Also, part of the superheated steam is jetted in an obliquely downward direction toward the door  11  (the direction indicated by arrow C) from the jet ports  67  formed in the front surface of the jet cover  61 . The steam present in the heating chamber  20  is sucked from the inlet port  28  by the blower fan  80 . In the blower fan  80 , the motor  81  drives the impeller  82  to rotate, to thereby suck the steam out from the heating chamber  20 . At this time, pressure inside the partition wall  27  becomes lower than outside the partition wall  27 , and thus, air tries to flow to the inside of the partition wall  27 . However, since the packing  83  is fitted into the through hole  27   a  and the sliding contact portion  83   b  is in sliding contact with the rotation shaft  81 , the through hole  27   a  is sealed. Thereby, outside air is prevented from flowing into the circulation duct  35 . 
     The flow of the superheated steam jetted frontward is bent by the sucking force from the blower fan  80 , to be led backward. Thereby, part of the superheated steam hits a front portion of the top surface of the article-to-be-heated F, and part of the superheated steam is led from the front side to below the article-to-be-heated F. As a result, the superheated steam is fully distributed in a front portion of the heating chamber  20 , and this prevents the front portion of the article-to-be-heated F from being undercooked, and thus the article-to-be-heated F can be uniformly cooked. 
     Also, since the superheated steam is sucked from the heating chamber  20  via the inlet port  28 , a smaller amount of the superheated steam, which is hot, directly hits the door  11 . This prevents the door  11  from being heated too much and thus eliminates the need of using a door with high resistance to heat as the door  11 , preventing cost increase of the steam cooking device  1 . 
     When an amount of steam present in the heating chamber  20  increases with time, excessive steam is exhausted from the device through the exhaust duct  32 . 
     The superheated steam jetted from the jet ports  65  and  67  is, after heating the article-to-be-heated, sucked from the inlet port  28  into the circulation duct  35  and flows into the steam heating device  40 . In this way, the steam inside the heating chamber  20  repeatedly circulates to during a cooking operation. Also, when the pot water-level detection portion  92  detects that the water level in the pot  50  has been lowered, the water supply pump  57  is driven to supply water to the pot  51  from the temporary reservoir  91 . 
     When the cooking is finished, it is indicated by a message displayed on a display portion of the operation panel  13  or by an audible alert. When the user, being notified that the cooking is finished, opens the door  11 , the damper  48  opens to forcibly exhaust the steam from inside the heating chamber  20  rapidly. This allows the user to safely take out the article-to-be-heated F from inside the heating chamber  20  without coming into contact with hot steam. 
     When the cooking is finished, the water drain valve  54  opens as well. This allows water remaining in the steam generating device  50  to be drained therefrom to flow through the water drain pipe  53  into the water tank  71  via the inlet port  71   a . The water remaining in the steam generating device  50  may be drained after it is naturally cooled down or forcibly cooled down by using a fan or the like. Furthermore, the user may be allowed to select between natural cooling down and forcible cooling down. 
     According to this embodiment, the grease G introduced to the inside of the packing  83  is scraped off by the sliding contact portion  83   b  and stored in the clearance  83   c . As a result, most part of the grease G applied to the peripheral surface of the rotation shaft  81   a  is stored inside the packing  83 . This helps reduce friction noise and friction heat caused by reduction of the grease G inside the packing  83  to an insufficient amount. Friction noise and friction heat can also be reduced by making the width of the sliding contact portion  83   b  in the axial direction smaller than that of the insertion hole  83   a  in the axial direction. 
     And, since friction noise and friction heat can be reduced in this way, the sliding contact portion  83   b  and the rotation shaft  81   a  can be designed to be in closer contact with each other. This helps prevent air from outside to inside of the partition wall  27 , to thereby improve airtightness. As a result, the heating chamber can be maintained in a super low-oxygen state where the oxygen concentration is on the order of 0.1% during a cooking operation. This helps prevent oxidation of the article-to-be-heated F to thereby prevent destruction of nutrients such as vitamin C. This further helps reduces oxidation of polyphenols, which have antioxidant characteristics and inhibit oxidation of accumulated bad LDL cholesterol, to thereby prevent disorders such as hyperpiesia, arteriosclerosis, and cerebrovascular disorder and cardiac diseases caused by arteriosclerosis. 
     Also, part of the sliding contact portion  83   b  outwardly protrudes in the diameter direction and the rotation shaft  81   a  is in sliding contact with the inner side of the sliding contact portion  83   b . This makes it possible for the sliding contact portion  83   b  to deal with the runout of the rotation shaft  81   a  by easily changing its shape on receiving contact pressure from the rotation shaft  81   a . This helps prevent a gap between the rotation shaft  81   a  and the sliding contact portion  83   b  without increasing the contact pressure, to thereby enhance the closeness of contact between the rotation shaft  81   a  and the sliding contact portion  83   b . This also helps reduce friction between the rotation shaft  81   a  and the sliding contact portion  83   b  when runout of the rotation shaft  81   a  has occurred. 
     The packing  83  may be fitted in a reverse direction, and a shaft extending from the impeller  82  may be inserted through the packing  83  from the end face  83   f  located on the side on which the clearance  83   c  is open, to be connected to the rotation shaft  81   a . With this structure, the packing  83  is disposed with the end face  83   f , which is located on the side on which the clearance  83   c  is open, facing the inner surface of the partition wall  27 . However, it is preferable that the end face  83   f  located on the side on which the clearance  83   c  is open face the inner surface of the partition wall  27  as in this embodiment. This is because, if the end face  83   f  located on the side on which the clearance  83   c  is open faces the outside of the partition wall  27 , the grease G stored in the clearance  83   c  can be prevented from coming into contact with steam. As a result, deterioration of the grease G due to steam can be prevented. 
     In this embodiment, the blower fan  80  is formed as what is called a sirocco fan provided with the impeller  82  having the plurality of blades  82   b  arranged along the outer periphery thereof, but a fan of a different type such as a propeller fan or a turbo fan may be used. The impeller  82  may be disposed inside the heating chamber  20 . In such a structure, a wall surface of the heating chamber  20  functions as a partition wall to isolate the motor  81  from circulating gas. 
     INDUSTRIAL APPLICABILITY 
     The present invention can be applied to a steam cooking device that jets steam into a heating chamber to thereby cook an article-to-be-heated and to a manufacturing method of such a steam cooking device. The present invention can also be applied to a packing with improved airtightness.