Abstract:
The invention provides a low-cost screw rotor that is capable of compressing gas. As exemplified in  FIG. 4 , screw rotors ( 26, 27 ) comprise: upstream screw section ( 43   a ) defined on the upstream side of the gas travel path with screw thread ( 41   a ) identically positioned at the same interval, and downstream screw section ( 43   b ) connecting to the downstream side of upstream screw section ( 43   a ) along the gas travel path, where threads ( 41   b ) on the downstream screw section are arranged continuously with identical pitch smaller than threads ( 41   a ) on the upstream screw section; and the upstream end of the downstream screw thread ( 41   b ) along the gas travel path and the downstream end of upstream screw thread ( 41   a ) along the gas travel path are continuously connected at a yielding point ( 43   c ).

Description:
TECHNICAL FIELD  
       [0001]     The invention relates to a screw rotor having a threaded outer periphery and a screw-type vacuum pump equipped with a pair of the screw rotors. In particular, the invention relates to a screw rotor that is capable of compressing gas during transportation of gas, and a vacuum pump equipped with the screw rotors.  
       BACKGROUND  
       [0002]     With respect to a vacuum pump that discharges gas from a vacuum chamber, a screw type vacuum pump that transports and discharges gas by means of rotation of jogged screw rotors having threaded outer periphery is well known. As an example, the herein described screw type dry vacuum pump applying traditional technology (J01) is well-known. (J01) Prior art is described in Patent document 1 (Japanese Patent Laid-open Publication No. 2000-45976)  
         [0003]     As described in Patent document 1, a screw-type dry vacuum pump comprises a pair of screw rotors having square thread on the outer periphery with identical lead (distance that a screw thread advances axially in one turn—on a single threaded screw, the lead and pitch are identical). (Japanese Patent Publication No. 2000-45976, paragraph 0022, FIG. 1)  
         [0004]     In the case of the screw pump applying traditional technology described in the prior art (J01), the screw rotors are designed to have identical lead (pitch) so that gas is discharged out of a discharge outlet from the upstream side without being compressed.  
         [0005]     Regarding compression of gas during gas transportation when screw rotors are used, a continuously variable pitch design of the screw rotor is considered wherein the pitch size at the suction side is maximum, and minimum at the discharge outlet. However, in the case of continuously variable pitch design, a high cost due to complicated machining is regarded as a problem.  
         [0006]     In addition, a small clearance is defined between threads of the two intermeshed screw rotors of the screw-type vacuum pump. When the threads have a rectangular tooth shape, they make interference (contact) on the surface of intermeshed threads as they rotate. As a result, the thread profile (interference prevention section) is machined to have a tapering (narrowing) end in order to prevent interference between threads in the case of the conventional screw type vacuum pump. However, since the intermesh interference between the threads becomes large with large lead size, it is required that the profile shape changes continuously with continuously variable pitch. In the case of continuously variable pitch of a thread, failure to carry out thread machining with one end-mill resulting in difficult machining and high cost.  
         [0007]     In addition, in order to compress the transported gas when screw rotors are used, in practical applications, the rotor of a large thread lead (large pitch) and the rotor of a small thread lead (pitch) are placed at a certain interval along the same revolving shaft. However, the discontinuous change of discharge volume between the rotor of a large thread lead and the rotor of a small thread lead results in low discharge efficiency. Moreover, because the two rotors are arranged at an interval, axial length of rotor (rotor length) tends to be increased. If the rotor length is increased, the dimension of the whole unit is forced to increase, together with the need for bearings to be mounted to support the revolving shaft at both ends as well as at the vacuum side. If bearings are mounted at the vacuum side, it becomes necessary to provide lubricant for the bearings; furthermore, measures should be taken to prevent pollution to the vacuum chamber resulting in a complicated structure and high cost.  
       SUMMARY OF INVENTION  
       [0008]     The invention provides a screw rotor at low cost that is capable of compressing air. The invention also increases discharge efficiency of a screw rotor and shortens the rotor length.  
         [0009]     In aspect  1  of the invention, a screw rotor is designed to be in possession of the following features. The screw rotor having threaded outer periphery rotates around the revolving shaft to transport gas. The screw rotor comprises two sections: an upstream screw section on the upstream side and a downstream screw section connecting the downstream side of the upstream screw section along the gas travel path.  
         [0010]     The threads on the upstream screw section are arranged at an identical pitch and the threads on the downstream screw section are arranged continuously at identical pitch smaller than that on the upstream screw section. The downstream end of the upstream screw threading along the gas travel path and the upstream end of the downstream screw threading along the gas travel path are continuously connected through inflexion points.  
         [0011]     The screw rotor further has threads on its outer periphery to rotate around a revolving shaft so as to transport gas. On the upstream side along the gas travel path, upstream threads are formed at identical intervals. On the downstream screw section connected with the downstream end of the upstream screw section along the gas travel path, the threads are formed continuously at identical interval smaller than that on the upstream screw section. Besides, in the downstream screw section, the downstream end of the upstream thread along the gas travel path and the upstream end of the downstream thread along gas travel path are continuously connected through inflexion points.  
         [0012]     Accordingly, both the upstream and downstream screw sections of the screw rotor in aspect  1  are formed at identical pitch, resulting in easy manufacture and low cost compared with the case of continuously variable pitch.  
         [0013]     Furthermore, because the pitch of the downstream screw section is smaller than that of the upstream section on the screw rotor in aspect  1 , gas is compressed as it travels from the upstream section to the downstream section. In the meantime, the downstream end of the upstream screw section and the upstream end of the downstream screw section are continuously connected at a yielding point, therefore, the gas discharge volume changes continuously near the yielding point. Compared to the screw rotor of variable pitch design whose gas discharge volume is not continuous, the screw rotor of the invention increases discharge efficiency and shortens the rotor length. In addition, the design of a shortened rotor length makes it easy to apply a stay bracket of bearings on one end of the revolving shaft.  
         [0014]     In Form  1  of aspect  1 , the screw rotor possesses the following features: it comprises an upstream interference prevention section on the upstream screw section and a downstream interference prevention section on the downstream screw section. The upstream interference prevention section on the upstream screw section is designed with tapering end of tread on upstream thread profile, and thread shape of the downstream interference prevention section on downstream thread profile of the downstream screw section is different from that of upstream interference prevention section.  
         [0015]     With respect to the screw rotor described in form  1  of aspect  1 , an upstream interference prevention section is designed on the thread profile of the upstream screw section with tapering end of thread, and similarly, a downstream interference prevention section is formed on the downstream thread profile of the downstream screw section that has thread shape different from that of upstream interference prevention section. Therefore, interference prevention areas are formed in order to prevent interference between intermeshed screw rotors.  
         [0016]     In aspect  2  of the invention, a vacuum pump is characterized by including the screw rotor in aspect  1  or form  1  of aspect  1 . Since the vacuum pump comprising the components in aspect  2  has the same screw rotors as that in aspect  1  or form  1  of aspect  1 , it has the same effect as aspect  1  or form  1  of aspect  1 .  
         [0017]     The foregoing described invention makes it possible to provide a screw rotor that is capable of compressing gas at low cost. Moreover, it is capable of increasing the discharge efficiency as well as shortening the rotor length. 
     
    
     DESCRIPTION OF THE FIGURES  
       [0018]      FIG. 1  is an overall illustration of screw type dry vacuum pump.  
         [0019]      FIG. 2  is an illustration of the screw rotor in embodiment  1 .  FIG. 2A  is the sectional view, and  FIG. 2B  the side view,  FIG. 2C  illustrates the view taken from the direction of arrow IIC in  FIG. 2B ,  FIG. 2D  illustrates the view taken from the direction of arrow IED in  FIG. 2B .  
         [0020]      FIG. 3  illustrates the main part of the screw rotor.  FIG. 3A  is a sectional view along line IIIA-IIIA of  FIG. 2B ,  FIG. 3B  shows an enlarged illustration of IIIA section in  FIG. 2A , and  FIG. 3C  shows an enlarged illustration of IIIC section in  FIG. 2A .  
         [0021]      FIG. 4  is the expanded view of the screw rotor profile in embodiment 1.  
         [0022]      FIG. 5  is an overall illustration of a screw type vacuum pump in embodiment 2, corresponding to  FIG. 1  of embodiment 1.  
         [0023]      FIG. 6  illustrates a screw rotor in embodiment 2 corresponding to  FIG. 2  in embodiment 1.  FIG. 6A  is a sectional view,  FIG. 6B  is a side view,  FIG. 6C  is a sectional view taken from arrow VIC of  FIG. 6B ,  FIG. 6D  is a sectional view taken from arrow VID of  FIG. 6B .  FIG. 6E  shows an enlarged illustration of VIE section in  FIG. 6A .  
         [0024]      FIG. 7  is an expanded view of the screw rotor profile in embodiment 2, corresponding to  FIG. 4  in embodiment 1. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     Various embodiments of application of the invention are illustrated in the accompanying drawings. It should be understood that applications of the invention are not limited to the following embodiments.  
         [0026]     Symbols used throughout in the Specification and the FIGS are explained as follows:  
         [0027]      1 . . . vacuum pump,  
         [0028]      21 ,  22 . . . revolving shafts,  
         [0029]      26 ,  27 ,  26 ′,  27 ′. . . screw rotor  
         [0030]      41   a  . . . upstream thread  
         [0031]      41   b  . . . downstream thread  
         [0032]      43  . . . screw  
         [0033]      43   a ,  43   a ′ . . . upstream screw section  
         [0034]      43   b ,  43   b ′ . . . downstream screw section  
         [0035]      43   c  . . . yielding point  
         [0036]      44   a  . . . upstream interference prevention section  
         [0037]      44   b  . . . downstream interference prevention section  
         [0038]     In order to make the illustration easily understood, the following definition is made to specify directions. X axis: the longitudinal (forward and aft), Y axis: the lateral (inward and outward) and Z axis: the vertical (up and down); and arrow marks X, -X, Y, -Y, Z,and -Z stand, respectively, for forward, aft, right, left, up and down in the drawings. In addition, the symbol ┌□┘ in ┌∘┘ represents an arrow directing out of the paper, and ┌x ┘ in ┌∘┘ represents an arrow directing into the paper.  
         [0039]     Embodiment 1 of the invention is further explained as follows.  FIG. 1  is an overall illustration of a screw type dry vacuum pump. It shows in  FIG. 1  that there is a base  2  mounted in the screw type dry vacuum pump  1  in the invention. The upper base  3  and lower base  4  are mounted on base  2 , with a gear room  6  designed within base  3  and base  4 . On the right side of base  2 , motor stand  3   a  to support pump motor M is mounted. A motor shaft through-hole  3   b  is drilled on motor stand  3   a  for motor shaft M 1  to go through. On the left side of the upper end of upper base  3 , a pair of upper bearing saddles  3   c  is mounted. On the left side of upper base  3 , discharge duct  7  is fixed, with which the downstream end of discharge passage  3   d  formed within base  3  is connected. Discharge outlet  3   e  is formed on the upstream end of discharge passage  3   d . On the upper end of the left side of lower base  4 , lower bearing saddle  4   a  is mounted. Lubricant for bearing lubrication is stored at the bottom of lower base  4 .  
         [0040]     To the left of the upper end of upper base  3 , a cylinder casing  11  is mounted. The top of casing  11  is blocked by cover  12 . On cover  12 , a suction inlet  12   a  while not shown in the drawings is connected with the vacuum chamber. Therefore, pump chamber  13  is defined in the internal space generated by the upper surface of upper base  3 , casing  11  and cover  12 , where suction inlet  12   a  is formed on the top and exhaust outlet  3   e  at the bottom. At the bottom of said casing  11  as well as around the outer periphery of upper base  13 , a doughnut-shaped cooling circuit  14  is mounted in which cooling water runs for cooling pump chamber  13 .  
         [0041]     Within base  2  as well as casing  11 , a pair of rotor axles (revolving shaft)  21 ,  22  are mounted at both left and right sides going through pump chamber  13  and gear room  6 . Rotor axles  21  and  22  are supported by upper bearing saddles  3   c  and lower bearing saddle  4   a  in gear room to rotate by means of bearings  23  and  24 . Within pump chamber  13  where rotor axles  21  and  22  are mounted, the intermeshed left and right screw rotors  26  and  27  are fixed. Within the bottom of screw rotors  26  and  27 , seal recesses  26   a  and  27   a  (referring to  FIG. 2 ) are designed. On the top of seal recesses  26   a  and  27   a , tapering axial through-holes  26   b  and  27   b  are made, and the tapering top of rotor axles  21  and  22  go through axial through-holes  26   b  and  27   b . At the top end of rotor axles  21  and  22 , screw threads are drilled and screwed tightly with nut N through washer W; therefore, rotor axles  21  and  22  are fixed together with screw rotors  26  and  27  thereby rotating integrally.  
         [0042]     Housing  28  for seal installation is fixed on the top of the upper bearing  23 . In seal recess  26   a , oil seal  29   a  fixed on seal installation housing  28 , together with the two flingers  29   b  and  29   c  that rotate with rotor axles  21  and  22 , comprises the seal system  29  functioning on prevention of leakage and reverse flow of gas and lubricant, etc.  
         [0043]     In gear room  6 , timing gears  31  and  32  are fixed to rotor axles  21  and  22  in an intermeshed state to transmit the driving power. In addition, in the vacuum pump described in embodiment 1, a rotation transmission gear  32   a  is fixed at the bottom of right timing gear  32 , and the right timing gear  32  is fixed rotationally with right rotor axle  22  integrally through the key and key groove that are not shown in the drawings. Furthermore, left timing gear  31  is connected with left rotor axle  21  through joint  33 . By means of joint  33 , the intermesh of gears  31  and  32  as well as that of screw rotors  26  and  27  is easily obtained.  
         [0044]     Drive gear  36  is fixed on the driving shaft M 1  of pump motor M. Rotation is transmitted to timing gears  31  and  32  through drive gear  36  together with the mid-gear  37  that is intermeshed with rotation transmission gear  32   a . In addition, bearing  38  that rotationally supports mid-gear  37 , together with other bearings  23  and  24 , are lubricated by the lubricant stored in lower base  4  through a lubricant supply device that is not shown in drawings.  
         [0045]      FIG. 2  is an illustration of the screw rotor in embodiment 1.  FIG. 2A  is the sectional view.  FIG. 2B  is a side view.  FIG. 2C  illustrates the view taken from the direction of arrow IIC in  FIG. 2B .  FIG. 2D  illustrates the view taken from the direction of arrow IID in  FIG. 2B .  
         [0046]      FIG. 3  illustrates the main part of the screw rotor.  FIG. 3A  is a sectional view taken along line IIIA-IIIA of  FIG. 2B .  FIG. 3B  shows an enlarged illustration of IIIA section in  FIG. 2A .  FIG. 3C  shows an enlarged illustration of IIIC section in  FIG. 2A .  
         [0047]      FIG. 4  is the expanded view of the screw rotor profile in embodiment 1. As show from  FIG. 2  to  FIG. 4 , one screw  43  with screw thread  41  and valley  42  is formed on the outer periphery of screw rotors  26  and  27  in embodiment 1. Screw  43  comprises upper upstream screw section  43   a  on the upstream side of the gas travel path and lower downstream strew section  43   b  on the downstream side of the gas travel path.  
         [0048]     In  FIG. 3B  and  FIG. 4 , adjacent upstream threads  41   a  are designed with identical distance (pitch p 1 , referring to  FIG. 3B ), with equal width of upstream screw valley  42   a  on upstream screw  43   a . In  FIG. 3C  and  FIG. 4 , adjacent downstream threads  42   a  are designed with identical distance (pitch p 2 , referring to  FIG. 3C ), with equal width of upstream screw valley  42   b  on downstream screw  43   b . Moreover, pitch p 2  (referring to  FIG. 3C ) is designed to be narrower in width compared with that of upstream screw  43   a.    
         [0049]     The downstream end of upstream screw section  43   a  and the upstream end of the downstream screw section  43   b  are continuously connected at yielding points  43   c  and  43   c  in  FIG. 4 . Accordingly, near the yielding points  43   c  and  43   c , the gas discharge volume changes smoothly in a continuous way, thus gas discharge volume changes continuously from upstream screw section  43   a  to the downstream screw section  43   b.    
         [0050]     In  FIG. 2C ,  FIG. 2D ,  FIG. 3B  and  FIG. 3C , on the end of thread profile of screw threads  41   a  and  42   a , interference prevention sections  44   a  and  44   b  are designed through cutting (chamfering machining) in order to remove interference when rotors  26  and  27  have rectangular teeth (which did not undergo chamfering). Moreover, with respect to interference prevention sections  44   a  and  44   b , since the shape of upstream interference prevention section  44   a  is different from that of the downstream interference prevention section  44   b , interference on the upstream side with a large lead tends to be large. Correspondingly, more cutting is required on the upstream interference prevention section  44   a  than on the downstream interference prevention section  44   b.    
         [0051]     In addition, on the surface near the root of screw threads  41   a  and  42   a , gap filling sections  46   a  and  46   b  are generated to fill the space between interference prevention sections  44   a  and  44   b  and the thread surface of screw threads  41   a  and  42   a  corresponding to the cutting of interference prevention sections  44   a  and  44   b . The existence of space between interference prevention sections  44   a  and  44   b  and the thread surface of screw threads  41   a  and  42   a  while rotors  26  and  27  are in intermeshed state will cause gas leaks and reverse flow; accordingly, discharge efficiency will decrease. Therefore, in embodiment 1, by means of clearance filling section  46   a  and  46   b , intermeshing is realized when rotors  26  and  27  are in intermeshed state; when the intermesh is released, interference generated is removed by interference prevention sections  44   a  and  44   b.    
         [0052]     In vacuum pump  1  having the structure described in embodiment 1, rotation is transmitted by gears  31 ,  32 ,  36  and  37  when pump motor M 1  starts; then rotor axles  21  and  22  start to rotate; furthermore, screw rotors  26  and  27  start to rotate. Accompanying the rotation of the screw rotors  26  and  27 , gas is transported by means of screw  43 . The gas inhaled from suction inlet  12   a  is discharged from discharge outlet  13 .  
         [0053]     On the screw rotors  26  and  27  defined in embodiment 1, upstream screw section  43   a  of a large lead (large pitch) on the upstream side is designed while downstream screw section  43   b  of a small lead (small pitch) on the downstream side is designed. Therefore, gas passing through upstream screw section  43   a  is compressed by means of a smaller gas discharge volume upon the arrival at downstream screw section  43   b . Besides, the fact that upstream screw section  43   a  and downstream screw section  43   b  are connected continuously at yielding point  43   c  leads to a continuously changeable gas discharge volume, in addition to the fact that the upstream screw section and downstream screw section are arranged with no space in between; therefore, gas discharge efficiency is superior compared with the case when the gas discharge volume changes in discontinuity.  
         [0054]     Furthermore, since the upstream screw section and downstream screw section are arranged with no space in between, rotor length can be shortened. Accordingly, the design of a stay bracket without the arrangement of a bearing on the vacuum side (on the side of suction inlet  12   a ) is applicable, as shown in  FIG. 1 .  
         [0055]     Moreover, corresponding to the degree of interference, interference prevention sections  44   a  and  44   b  are arranged on screw rotors  26  and  27  to remove interference when rotors are released from intermesh. In addition, the advantage that space is filled by means of gap filling sections  46   a  and  46   b  leads to reduction of gas leakage and increase of discharge efficiency.  
         [0056]     In addition, for screw rotors  26  and  27  defined in embodiment 1, it is possible to make screw rotors  26  and  27  as follows: the screw thread higher than the yielding point  43   c  is machined by a large lead cutting tool, and the screw thread lower than the yielding point  43   c  is machined by a small lead cutting tool. Therefore, compared with the case that the pitch of the screw thread is continuously variable, machining of screw rotors  26  and  27  is easier. In addition, compared with the case of continuously variable shape, it is easy to make interference prevention sections  44   a  and  44   b  since they have two different shapes, resulting in low cost.  
         [0057]     Embodiment 2 of the invention is further explained as follows.  FIG. 5  is an overall illustration of screw type vacuum pump in embodiment 2, corresponding to  FIG. 1  embodiment 1.  FIG. 6  illustrates screw rotor in embodiment 2 corresponding to  FIG. 2  of embodiment 1.  FIG. 6A  is a sectional view.  FIG. 6B  is a side view.  FIG. 6C  is a sectional view taken from arrow VIC  FIG. 6B .  FIG. 6D  is a sectional view taken from arrow VID of  FIG. 6B .  FIG. 6E  shows an enlarged illustration of VIE section in  FIG. 6A .  
         [0058]      FIG. 7  is the expanded view of the screw rotor profile in embodiment 2, corresponding to  FIG. 4  in embodiment 1. Additionally, regarding the illustration of embodiment 2, same symbols for the corresponding components are used as in embodiment 1 whith specific explanation of the symbols omitted. Regarding the components of embodiment 2, it is the same as those of embodiment 1 except the following features.  
         [0059]     As shown in  FIG. 5  to  FIG. 7 , screw type vacuum pump  1  of embodiment 2 is equipped with screw rotors  26 ′ and  27 ′ which are different from the screw rotors  26  and  27  of the embodiment 1. In screw rotors  26 ′ and  27 ′ of the embodiment 2, an innite lead section  61  is formed continuously, on the upstream side of gas travel path, further up than upstream screw section  43   a  (the side of suction inlet  12   a ), with a length corresponding to thread lead infinitely great. Infinite lead section  61  comprises small circle section  61   a  and large circle section  61   b . On large circle section  61   b , a circular excision  61   c  is formed. In addition, gas compression section  61   d  is formed between small circle section  61   a  and large circle section  61   b . As described in patent document 1 (referring to rotor extensions  7  and  8  in patent document 1), being normally well-known, infinite lead section  61  is applied to discharge gas by a discharge mechanism depicted in  FIG. 3  of Patent document 1, therefore, detailed illustration is omitted. Additionally, in the structure described in patent document 1, the center of gravity of infinite rotor section  61  deviates. It becomes ill-balanced as screw rotors  26 ′ and  27 ′ rotate. In embodiment 2 of the invention, excision  61   c  is applied on infinite rotor section  61  to keep screw rotors  26 ′ and  27 ′ in a balanced state.  
         [0060]     In  FIG. 6  and  FIG. 7 , the downstream end of upstream screw section  43   a ′ is continuously connected with the upstream end of downstream screw section  43   b ′ at yielding points  43   c  and  43   c ; in addition, the upstream end of upstream screw  43   a ′ is continuously connected with the downstream end (lower end) of infinite lead section  61  at yielding point  43   c ′ and  43   c ′ in screw rotors  26 ′ and  27 ′.  
         [0061]     Furthermore, in screw rotors  26 ′ and  27 ′of the embodiment 2, more thread turns of downstream screw section  43   b ′ (7 turns) are designed compared with that in the case of embodiment 1.  
         [0062]     In the screw type vacuum pump  1  that has the structure defined in embodiment 2, gas is compressed as it travels from infinite lead section  61  to upstream screw section  43   a ′ as well as from upstream screw section  43   a ′ to downstream screw section  43   a ′. Therefore, compression ratio is increased.  
         [0063]     In addition, in screw rotors  26 ′ and  27 ′of the embodiment 2, the fact that infinite lead section  61  is continuously connected with upstream end of upstream screw section  43   a ′ at yielding point  43   c ′ with no space in between, together with the fact that the upstream screw section  43   a ′ and downstream screw section  43   b ′ are continuously connected at yielding point  43   c , make it possible for the rotor length of screw rotors  26  and  27  to be shortened. Consequently, application of stay bracket is easy to accomplish. Furthermore, continuous connection enables continuously changeable discharge volume resulting in increased discharge efficiency.  
         [0064]     Furthermore, with respect to screw rotors  26 ′ and  27 ′of the embodiment 2, more thread turns of downstream screw section  43   b ′ of small lead are designed so that transported gas is separated into more parts. Accordingly, it can reduce leakage from discharge outlet  3   e  to suction inlet.  
         [0065]     Apart from the foregoing description, vacuum pump  1  of embodiment 2 has similar effect to that of embodiment 1. Some embodiments of the invention have been the described in detail, but it is to be understood that the invention is not limited exclusively to the described embodiments, within the scope of the claims of the invention, variations can be made Variations (H01) to (H06) of the invention are illustrated below.  
         [0066]     (H01) In the embodiments, it is possible to install additional one or more screw threads on the upstream side of upstream screw sections  43   a  and  43   a ′ or the downstream side of downstream screw sections  43   b  and  43   b ′. That is, it is possible to design a screw rotor with more than three screw sections instead of 2.  
         [0067]     (H02) In the embodiments, it is preferred that screw  43  comprises one thread; however, screw  43  may comprise more than two threads.  
         [0068]     (H03) In various embodiments, the specific numbers of thread turns of screws and the length of lead, p 1 , p 2 , p 1 ′ and p 2 ′, as well as the rotor length, can be varied based on specific designs.  
         [0069]     (H04) In the embodiments, it is possible to install roots rotor instead of infinite lead  61  as described in patent document 1.  
         [0070]     (H05) In the embodiments, the threads are designed to be curved and continuously connected at yielding point  43   c  and  43   c ′. In addition, it should be understood that it is also possible to cut off the curved angle so as to connect smoothly near the yielding point.  
         [0071]     (H06) In the embodiments, the driving force of pump motor M that is supported by motor stand  3   a  is transmitted to rotor axles  21  and  22  through gear  36  and mid-gear  37 ; however, it is not limited to the present structure. Either of rotor axles  21  and  22  can support the motor rotor with a motor starter located around the rotor; that is to say, a built-in motor structure can be applied. Incorporation of the built-in motor results in a compact structure and reduced cost.