Patent Publication Number: US-9841023-B2

Title: Vacuum pump

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is the U.S. National Phase of PCT/JP2013/064113 filed May 21, 2013, which claims priority to Japanese Patent Application No. 2012-115804 filed May 21, 2012, which claims priority to Japanese Patent Application No. 2012-116479 filed May 22, 2012. The disclosures of the above applications are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a vacuum pump having a rotor secured to a rotating shaft of a driving machine. 
     BACKGROUND ART 
     There is generally known a vacuum pump having a casing body secured to a driving machine, a hollow cylinder chamber which is formed in the casing body and has an opening at an end portion of the casing body, a rotor which is rotationally driven in the cylinder chamber, a side plate which blocks the opening of the cylinder chamber, and a pump cover which is disposed at the opposite side of the rotor so as to sandwich the side plate between the pump cover and the rotor and fixed to the casing body. This type of vacuum pump is used to generate vacuum for actuating a power braking device of a vehicle, for example, and it can obtain vacuum by driving a rotor in a cylinder chamber of a casing with a driving machine such as an electric motor or the like (see Patent Document 1, for example). 
     PRIOR ART DOCUMENT 
     
         
         Patent Document 1: U.S. Pat. No. 6,491,501 
       
    
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     In the conventional construction, the space formed between the side plate and the pump cover is under ambient pressure, whereas the vicinity of a shaft hole of the rotor which faces the side plate intercommunicates with a space under negative pressure occurring during operation of the vacuum pump through the gap between the rotor and the side plate, so that the vicinity of the shaft hole is set to ambient pressure or less (that is, negative pressure) in some cases. 
     Therefore, for example when the side plate is formed of a material having low rigidity such as carbon or the like, the side plate sags due to pressure difference, and the rotor and the side plate are brought into contact with each other during operation of the vacuum pump. Therefore, there has been assumed a problem that the rotor and the side plate are worn away and the durability of the vacuum pump is degraded. 
     The present invention has been implemented in view of the foregoing situation, and has an object to suppress abrasion of a rotor and a side plate with a simple construction, thereby preventing degradation of durability of a vacuum pump. 
     Means of Solving the Problem 
     In order to attain the above object, according to the present invention, a vacuum pump including a casing body having a hollow cylinder chamber opened at an end portion thereof, a rotor rotated in the cylinder chamber, a side plate which blocks the opening of the cylinder chamber, and a pump cover which is disposed at the opposite side to the rotor so as to sandwich the side plate between the pump cover and the rotor and fixed to the casing body, is characterized in that the side plate is provided with an intercommunication port that faces a shaft hole of the rotor and intercommunicates with a space between the side plate and the pump cover. 
     According to this construction, the side plate is provided with the intercommunication port which confronts the shaft hole of the rotor and intercommunicates with the space between the side plate and the pump cover, and thus the pressure difference between the neighborhood of the shaft hole of the rotor and the space can be suppressed. Therefore, the contact between the rotor and the side plate can be prevented, whereby the abrasion of the rotor and the side plate can be suppressed and the durability of the vacuum pump can be enhanced. 
     In this construction, the intercommunication port may be formed to be smaller than the shaft diameter of the rotating shaft for rotating the rotor. According to this construction, the amount of air flowing through the intercommunication port can be suppressed, and thus the compressibility when the rotor is rotated can be prevented from being reduced, so that degradation of the performance of the vacuum pump can be prevented. 
     Furthermore, the intercommunication port may be formed on the axial center of the shaft hole of the rotor. According to this construction, the intercommunication port is provided at the position which has the least influence on compression and expansion when the rotor is rotated. Therefore, the reduction of the compressibility when the rotor is rotated can be prevented, and the degradation of the performance of the vacuum pump can be prevented. 
     Furthermore, a seal member through which an exhaust passage from the cylinder chamber to the outside thereof and the space are isolated from each other may be disposed around the cylinder chamber between the casing body and the pump cover. According to this construction, exhausted air can be prevented from flowing into the space by the seal member, and thus the contact between the rotor and the side plate can be surely prevented. 
     According to the present invention, a vacuum pump having a rotating and compressing element driven by a motor in a casing is characterized in that the casing has a cylinder liner in which the rotating and compressing element slides, and a bearing portion for supporting a rotating shaft of the motor, and is secured to an opening portion of a cylindrical motor case body having a bottom. 
     According to this construction, the casing has the cylinder liner in which the rotating and compressing element slides, and the bearing portion for supporting the rotating shaft of the motor, and is secured to the opening portion of the cylindrical motor case body having the bottom. Therefore, the positional relationship between the cylinder liner and the rotating and compressing element can be regulated by only the casing. Therefore, misalignment occurring when the casing and the electric motor are assembled can be suppressed, and substantially uniform performance can be exercised with little individual difference. Furthermore, the casing can be formed by a single mold, so that the number of parts can be reduced and the manufacturing cost can be reduced. 
     In this construction, the casing has the bore portion in which the cylinder liner is disposed, and the bore portion may be a stepped bore which is reduced in diameter from the open end to the depth side. According to this construction, when the cylinder liner is disposed in the bore portion, the cylinder liner can be easily positioned because the end portion of the cylinder liner abuts against the step portion of the stepped bore. 
     The bore diameter of the diameter-reduced portion of the stepped bore may be set to be larger than the inner diameter of the cylinder liner. According to this construction, the side plate which is larger than the inner diameter of the cylinder liner can be disposed at the diameter-reduced portion, and the opening of the cylinder liner can be easily blocked by the side plate. 
     Effect of the Invention 
     According to the present invention, the side plate is provided with the intercommunication port which confronts the shaft hole of the rotor and intercommunicates with the space between the side plate and the pump cover, and thus the pressure difference between the neighborhood of the shaft hole of the rotor and the space can be suppressed. Therefore, the contact between the rotor and the side plate is prevented, whereby the abrasion of the rotor and the side plate can be suppressed and the durability of the vacuum pump can be enhanced. 
     According to the present invention, the casing has the cylinder liner in which the rotating and compressing element slides, and the bearing portion for supporting the rotating shaft of the motor, and is secured to the opening portion of the cylindrical motor case body having the bottom. Therefore, the positional relationship between the cylinder liner and the rotating and compressing element can be regulated by only the casing. Therefore, misalignment occurring when the casing and the electric motor are assembled can be suppressed, and substantially uniform performance can be exercised with little individual difference. Furthermore, the casing can be formed by a single mold, so that the number of parts can be reduced and the manufacturing cost can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a brake device using a vacuum pump according to an embodiment. 
         FIG. 2  is a partially sectional view of a side portion of the vacuum pump; 
         FIG. 3  is a diagram showing the vacuum pump when the vacuum pump is viewed from the front side thereof. 
         FIG. 4  is a partially enlarged view of  FIG. 2 . 
         FIG. 5  is a diagram showing the relationship between the shaft center of the rotor and the side plate. 
         FIG. 6  is a partially sectional view of a side portion of the vacuum pump according to a second embodiment. 
         FIG. 7  is a diagram showing the vacuum pump when the vacuum pump is viewed from the rear side thereof. 
         FIG. 8  is a partially enlarged view of  FIG. 6 . 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Preferred embodiments according to the present invention will be described hereunder with reference to the accompanying drawings. 
     First Embodiment 
       FIG. 1  is a diagram showing a brake device  100  in which a vacuum pump  1  according to an embodiment of the present invention is used as a negative pressure source. The brake device  100  has front brakes  2 A,  2 B secured to the right and left front wheels of a vehicle such as a car or the like, and rear brakes  3 A,  3 B secured to the right and left rear wheels. Each of these brakes is connected to a master cylinder  4  and a brake pipe  9 , and actuated with hydraulic pressure fed from the master cylinder  4  through the brake pipe  9 . 
     Furthermore, the brake device  100  has a brake booster (power braking device)  6  connected to the brake pedal  5 , and a vacuum tank  7  and the vacuum pump  1  are connected to the brake booster  6  through an air pipe  8  in series. The brake booster  6  boosts tread force of a brake pedal  5  by using the negative pressure in the vacuum tank  7 , and it is configured to derive sufficient brake force by moving a piston (not shown) of the master cylinder  4  with small tread force. 
     The vacuum pump  1  is disposed in an engine room of the vehicle, and it discharges air in the vacuum tank  7  to the outside of the vehicle to set the inside of the vacuum tank  7  to a vacuum state. The use range of the vacuum pump  1  used for a car or the like is from −60 kPa to −80 kPa, for example. 
       FIG. 2  is a partially sectional view of the side portion of the vacuum pump  1 , and  FIG. 3  is a diagram showing the vacuum pump  1  when the vacuum pump  1  of  FIG. 2  is viewed from the front side thereof (the right side in  FIG. 2 ). However,  FIG. 3  shows a state that the members such as the pump cover  24 , the side plate  26 , etc. are detached to show the construction of a cylinder chamber S. In the following description, the directions represented by arrows at the upper portion of  FIGS. 2 and 3  represent upper, lower, front, rear, right and left sides of the vacuum pump  1  for convenience of description. The front-and-rear direction is also referred to as “axial direction”, and the right-and-left direction is also referred to as “width direction”. 
     As shown in  FIG. 2 , the vacuum pump  1  has an electric motor (driving machine)  10 , and a pump body  20  which is actuated by the electric motor  10  as a driving source. The vacuum pump  1  is fixed and supported in a vehicle body such as a car or the like while the electric motor  10  and the pump body  20  are integrally connected to each other. 
     The electric motor  10  has an output shaft (rotating shaft)  12  which extends from substantially the center of one end portion (front end) of a case  11  configured in a substantially cylindrical shape to the pump body  20  side (front side). The output shaft  12  functions as a driving shaft for driving the pump body  20 , and rotates around the rotational center X1 extending in the front-and-rear direction. A rotor  27  of the pump body  20  is integrally rotatably connected to a tip portion  12 A of the output shaft  12 . 
     When the electric motor  10  is powered by a power source (not shown), the output shaft  12  rotates in the direction of an arrow R (counterclockwise) in  FIG. 3 , whereby the rotor  27  is rotated in the same direction (the direction of the arrow R) around the rotational center X1. 
     The case  11  has a case body  60  having a bottom which is configured in a cylindrical shape, and a cover body  61  for blocking the opening of the case body  60 . The case body  60  is configured so that the peripheral edge portion  60 A of the opening is bent outwards. The cover body  61  has a disc plate portion  61 A which is formed to have substantially the same diameter as the opening of the case body  60 , a cylindrical portion  61 B which annually extends from the peripheral edge of the disc plate portion  61 A in the axial direction and is fitted to the inner peripheral surface of the case body  60 , and a bent portion  61 C which is formed by bending the peripheral edge of the cylindrical portion  61 B outwards, the disc plate portion  61 A, the cylindrical portion  61 B and the bent portion  61  being formed integrally with one another. The disc plate portion  61 A and the cylindrical portion  61 B enter the inside of the case body  60 , and the bent portion  61 C is fixed in contact with the peripheral edge portion  60 A of the case body  60 . Accordingly, in the electric motor  10 , one end portion (front end) of the case  11  is recessed inwards, and a fitting bore portion  63  to which the pump body  20  is faucet-fitted is formed. 
     A through hole  61 D through which the output shaft  12  penetrates, and an annular bearing holding portion  61 E extending to the inside of the case body  60  around the through hole  61 D are formed substantially at the center of the disc plate portion  61 A, and an outer ring of the bearing  62  which pivotally supports the output shaft  12  is held by the inner peripheral surface  61 F of the bearing holding portion  61 E. 
     As shown in  FIG. 2 , the pump body  20  has the casing body  22  fitted in the fitting bore portion  63  formed at the front side of the case  11  of the electric motor  10 , a cylinder portion  23  which is integrally casted in the casing body  22  to form a cylinder chamber S, and a pump cover  24  which covers the casing body  22  from the front side. In this embodiment, a casing  31  of the vacuum pump  1  is constructed to have the casing body  22 , the cylinder portion  23  and the pump cover  24 . 
     The casing body  22  is formed of metal material having high thermal conductivity such as aluminum or the like and configured in a substantially rectangular shape which is longer in the up-and-down direction with the rotational center X1 being located substantially at the center of the shape in front view. An intercommunication hole  22 A which intercommunicates with the cylinder chamber S provided to the casing body  22  is formed at the upper portion of the casing body  22 , and a vacuum suction nipple  30  is press-fitted in the intercommunication hole  22 A. As shown in  FIG. 2 , the vacuum suction nipple  30  is a straight pipe extending upwards, and a pipe or tube for supplying negative-pressure air from external equipment (for example, the vacuum tank  7  (see  FIG. 1 )) is connected to one end  30 A of the vacuum suction nipple  30 . 
     A hole portion  22 B extending in the front-and-rear direction is formed in the casing body  22  based on an axial center X2, and the cylinder portion  23  formed in a cylindrical shape is integrally casted in the hole portion  22 B. Specifically, under the state that the cylinder portion (cylinder liner)  23  is set in a mold, teeming into the mold is performed to cast the casing body  22  (casing  31 ) in which the cylinder portion  23  is integrally casted. In this embodiment, the cylinder portion  23  is integrally casted in the casing body  22 . However, the present invention is not limited to this style, and the cylinder portion  23  may be press-fitted in the hole portion  22 B of the casing body  22  which has been casted in advance. 
     The axial center X2 is parallel to the rotational center X1 of the output shaft  12  of the electric motor  10 , and eccentrically displaced from the rotational center X1 to the upper left side as shown in  FIG. 2 . In this construction, the axial center X2 is eccentrically displaced so that the outer peripheral surface  27 B of the rotor  27  having the rotational center X1 as the center makes contact with the inner peripheral surface  23 A of the cylinder portion  23  which is formed based on the axial center X2. 
     The cylinder portion  23  is formed of the same metal material (iron in this embodiment) as the rotor  27 . In this construction, the cylinder portion  23  and the rotor  27  have the same thermal expansion coefficient. Therefore, the contact between the outer peripheral surface  27 B of the rotor  27  and the inner peripheral surface  23 A of the cylinder portion  23  when the rotor  27  is rotated can be prevented irrespective of temperature variation of the cylinder portion  23  and the rotor  27 . The cylinder portion  23  and the rotor  27  may be formed of different materials insofar as these materials are metal materials having substantially the same thermal expansion coefficient. 
     The cylinder portion  23  is integrally casted in the hole portion  22 B formed in the casing body  22 , whereby the cylinder portion  23  can be accommodated within the length range of the casing body  22  in the front-and-rear direction. Therefore, the cylinder portion  23  can be prevented from protruding from the casing body  22 , and the casing body  22  can be miniaturized. 
     Furthermore, the casing body  22  is formed of a material having higher thermal conductivity than the rotor  27 . Accordingly, heat occurring when the rotor  27  and vanes  28  are rotated can be quickly transferred to the casing body  22 , so that heat can be sufficiently radiated from the casing body  22 . 
     An opening  23 B through which the intercommunication hole  22 A of the casing body  22  intercommunicates with the cylinder chamber S is formed in the cylinder portion  23 , and air passing through the vacuum suction nipple  30  is supplied through the intercommunication hole  22 A and the opening  23 B into the cylinder chamber S. Therefore, in this embodiment, an air-intake passage  32  is configured to have the vacuum suction nipple  30 , the intercommunication hole  22 A of the casing body  22  and the opening  23 B of the cylinder portion  23 . Discharge ports  22 C,  23 C which penetrate through the casing body  22  and the cylinder portion  23  and through which air compressed in the cylinder chamber S is discharged are provided at the lower portions of the casing body  22  and the cylinder portion  23 . 
     Side plates  25 ,  26  for blocking the openings of the cylinder chamber S are disposed at the rear and front ends of the cylinder portion  23 . These side plates  25 ,  26  are configured so that the diameters thereof are larger than the inner diameter of the inner peripheral surface  23 A of the cylinder portion  23 , and urged to be pressed against the front end and rear end of the cylinder portion  23  by seal rings  25 A,  26 A. Accordingly, the cylinder chamber S which is hermetically closed except for the opening  23 B intercommunicating with the vacuum suction nipple  30  and the discharge ports  23 C,  22 C is formed inside the cylinder portion  23 . 
     The rotor  27  is disposed in the cylinder chamber S. The rotor  27  has a columnar shape extending along the rotational center X1 of the electric motor  10 , and has a shaft hole  27 A in which the output shaft  12  as the driving shaft of the pump body  20  is inserted. Plural guide grooves  27 C are provided at positions of the rotor  27  which are away from the shaft hole  27 A in the radial direction and spaced from one another at regular angular intervals in the peripheral direction around the shaft hole  27 A. 
     The length in the front-and-rear direction of the rotor  27  is set to be substantially equal to the length of the cylinder chamber S of the cylinder portion  23 , that is, the distance between the confronting inner surfaces of the two side plates  25 ,  26 , and the gap between the rotor  27  and each of the side plates  25 ,  26  is substantially closed. 
     The outer diameter of the rotor  27  is set so that the outer peripheral surface  27  of the rotor  27  keeps a minute clearance from a portion of the inner peripheral surface  23 A of the cylinder portion  23  which is located at the lower right position as shown in  FIG. 3 . Accordingly, as shown in  FIG. 3 , a crescent-shaped space is formed between the outer peripheral surface  27 B of the rotor  27  and the inner peripheral surface  23 A of the cylinder portion  23 . 
     The rotor  27  is provided with plural (five in this embodiment) vanes  28  for sectioning the crescent-shaped space. The vane  28  is formed like a plate, and the length of the vane  28  in the front-and-rear direction is set to be substantially equal to the distance between the confronting inner surfaces of the two side plates  25 ,  26  as in the case of the rotor  27 . These vanes  28  are disposed to freely protrude from and retract into the guide grooves  27 C provided to the rotor  27 . Each vane  28  protrudes outwards along the guide groove  27 C by centrifugal force in connection with the rotation of the rotor  27 , and the tip thereof is brought into contact with the inner peripheral surface  23  of the cylinder portion  23 . Accordingly, the crescent-shaped space described above is sectioned into five compression chambers P which are surrounded by the respective adjacent two vanes  28 ,  28 , the outer peripheral surface  27 B of the rotor  27  and the inner peripheral surface  23 A of the cylinder portion  23 . In connection with the rotation of the rotor  27  in the direction of the arrow R which is caused by the rotation of the output shaft  12 , these compression chambers P rotate in the same direction, and the volume thereof increases in the neighborhood of the opening  23 B while the volume thereof decreases at the discharge port  23 C. That is, through the rotation of the rotor  27  and the vanes  28 , air sucked from the opening  23 B into one compression chamber P is compressed and discharged from the discharge port  23 C while circulating in connection with the rotation of the rotor  27 . 
     In this construction, the cylinder portion  23  is formed in the casing body  22  so that the axial center X2 of the cylinder portion  23  is eccentrically displaced to the upper left side with respect to the rotational center X1 as shown in  FIG. 2 . Therefore, in the casing body, a large space can be secured in the opposite direction to the eccentric displacement direction of the cylinder portion  23 , and an expansion chamber  33  intercommunicating with the discharge ports  23 C,  22 C is formed along the peripheral edge portion of the cylinder portion  23  at this space. 
     The expansion chamber  33  is formed as a large closed space which expands along the peripheral edge portion of the cylinder portion  23  from the lower side of the cylinder portion  23  to the upper side of the output shaft  12 , and intercommunicates with an exhaust port  24 A formed in the pump cover  24 . The compressed air flowing into the expansion chamber  33  expands and disperses in the expansion chamber  33 , impinges against the partition wall of the expansion chamber  33  and irregularly reflects from the partition wall. Accordingly, the sound energy of the compressed air is attenuated, so that noise and vibration occurring when the compressed air is exhausted can be reduced. In this embodiment, an exhaust passage  37  is configured to have the discharge ports  22 C,  23 C formed in the casing body  22  and the cylinder portion  23  respectively, the expansion chamber  33  and the exhaust port  24 A. 
     In this embodiment, the cylinder portion  23  is disposed to be eccentrically displaced from the rotational center X1 of the rotor  27 , whereby a large space can be secured at the peripheral edge portion at the rotational center X1 side of the cylinder portion  23  in the casing body  22 . Therefore, the expansion chamber  33  can be integrally formed in the casing body  22  by forming the large expansion chamber  33  in this space, so that it is unnecessary to provide the expansion chamber  33  at the outside of the casing body  22  and the casing body  22  can be miniaturized, and further the vacuum pump  1  can be miniaturized. 
     The pump cover  24  is disposed on the front-side side plate  26  through the seal ring  26 A, and fixed to the casing body  22  by bolts  66 . As shown in  FIG. 2 , a seal groove  22 D is formed on the front surface of the casing body  22  so as to surround the cylinder portion  23  and the expansion chamber  33 , and an annular seal member  67  is disposed in the seal groove  22 D. The pump cover  24  is provided with an exhaust port  24 A at the position corresponding to the expansion chamber  33 . The exhaust port  24 A serves to discharge the air flowing in the expansion chamber  33  to the outside of the machine (the outside of the vacuum pump  1 ), and a check valve  29  for preventing flowback of air from the outside of the machine into the pump is secured to the exhaust port  24 A. 
     As described above, the vacuum pump  1  is constructed by connecting the electric motor  10  and the pump body  20 , and the rotor  27  connected to the output shaft  12  of the electric motor  10  and the vanes  28  slide in the cylinder portion  23  of the pump body  20 . Therefore, it is important to assemble the pump body  20  in conformity with the rotational center X1 of the output shaft  12  of the electric motor  10 . 
     Therefore, in this embodiment, the electric motor  10  has the fitting bore portion  63  which is formed at one end side of the case  11  with the rotational center X1 of the output shaft  12  at the center thereof. Furthermore, as shown in  FIG. 2 , a cylindrical fitting portion  22 F which projects rearwards around the cylinder chamber S is formed integrally with the back surface of the casing body  22 . The fitting portion  22 F is formed concentrically with the rotational center X1 of the output shaft  12  of the electric motor  10 , and configured to have such a diameter that the fitting portion  22 F is faucet-fitted to fitting bore portion  63  of the electric motor  10 . 
     Therefore, in this construction, centering can be simply performed by merely fitting the fitting portion  22 F of the casing body  22  into the fitting bore portion  63  of the electric motor  10 , and an assembling work for the electric motor  10  and the pump body  20  can be easily performed. Furthermore, a seal groove  22 E is formed around the fitting portion  22 F on the back surface of the casing body  22 , and an annular seal member  35  is disposed in the seal groove  22 E. 
     Next, a connection structure for the rotor  27  and the output shaft  12  will be described. 
     A male screw (not shown) is formed on the tip portion  12 A of the output shaft  12 , and this male screw is engaged with a female screw (not shown) which is formed at a part of the shaft hole  27 A penetrating through the rotor  27  in the axial direction thereof, whereby the output shaft  12  and the rotor  27  are connected to each other to be integrally rotatable. Furthermore, a nut  70  is engaged with the male screw of the output shaft  12  at the tip (side plate  26 ) side of the rotor  27 , thereby restricting movement of the rotor  27  to the tip side of the output shaft  12 . 
     As shown in  FIG. 4 , the output shaft  12  is formed so that the tip portion  12 A thereof is smaller in diameter than the base portion  12 C thereof, and a male screw is formed on the outer peripheral surface of the diameter-reduced tip portion  12 A. 
     On the other hand, the shaft hole  27 A of the rotor  27  has a shaft holding portion  27 E in which the base portion  12 C of the output shaft  12  is fitted, a hole portion  27 F smaller in diameter than the shaft holding portion  27 E and a recess portion  27 H larger in diameter than the hole portion  27 F and the shaft holding portion  27 E, and a female screw is formed on the inner peripheral surface of the hole portion  27 F. The shaft holding portion  27 E is formed to be longer in the shaft direction than the hole portion  27 F having the female screw, and specifically it is longer than the half of the whole length of the rotor  27 . The shaft holding portion  27 E is formed to be substantially equal in diameter to the base portion  12 C of the output shaft  12 . Accordingly, the rotor  27  is fitted to the base portion  12 C of the output shaft  12  over the half of the whole length or more, and thus the rotor  27  is prevented from being tilted. 
     The recess portion  27 H is opened to the front end surface  27 G of the rotor  27 , the tip portion of the male screw of the output shaft  12  extends into the recess portion  27 , and the nut  70  is engaged with the male screw in the recess portion  27 H. In this embodiment, the length of the shaft end of the output shaft  12  extending to the inside of the recess portion  27 H and the thickness of the nut  70  are set to be substantially equal to or slightly smaller than the depth of the recess portion  27 H, whereby the output shaft  12  and the nut  70  are prevented from protruding from the front end face  27 G of the rotor  27 . Furthermore, the inner diameter of the recess portion  27 H is set to such a size that the nut  70  disposed in the recess portion  27 H can be fastened by a tool (for example, socket wrench or the like). 
     In this construction, the female screw of the rotor  27  and the female screw of the nut  70  are engaged with the male screw of the output shaft  12 , whereby the rotor  27  and the nut  70  exercise a so-called double nut effect. Therefore, the rotor  27  is restricted from moving in the radial direction and the thrust direction with respect to the output shaft  12 , whereby the contact between the rotor  27  and the side plates  25 ,  26  can be prevented with a simple construction, and abrasion of the rotor  27  and the side plates  25 ,  26  can be suppressed and the durability of the vacuum pump  1  can be enhanced. 
     Furthermore, in this construction, the male screw of the output shaft  12  is formed as a left-hand screw (reverse screw), and the rotor  27  is connected to the output shaft  12  by rotating the rotor  27  in the same direction as the output shaft  12  (counterclockwise) when the pump is viewed from the front side. In this construction, force acts on the rotor  27  in such a direction that the rotor  27  is screwed into the output shaft  12  every time the vacuum pump  1  is stopped, and thus the rotor  27  and the nut  70  can be prevented from slacking in even a machine which repeats actuation and stop such as the vacuum pump  1 . 
     In this type of vacuum pump, air in the exhaust passage  37  infiltrates into the space  80  formed between the side plate  26  at the front side and the pump cover  24  through the gap between the casing body  22  and the pump cover  24 , so that the space  80  is set to the atmospheric pressure. Furthermore, the shaft hole  27 A of the rotor  27  facing the side plate  26  intercommunicates with the space (the air-intake passage  32 ) under negative pressure occurring during operation of the vacuum pump  1  through the gap between the rotor  27  and the side plate  26 , whereby the inside of the shaft hole  27 A is set to the atmospheric pressure or less (that is, the negative pressure). 
     Since the side plate  26  is formed of a material having low rigidity such as carbon or the like in this construction, the side plate  26  slacks due to the pressure difference, and the rotor  27  and the side plate  26  come into contact with each other during operation of the vacuum pump  1 . Therefore, there may occur a problem that the side plate  26  is worn away and thus the durability of the vacuum pump  1  is degraded. 
     Accordingly, according to this construction, an intercommunication port  261  which faces the shaft hole  27 A of the rotor  27  and intercommunicates with the space  80  between the side plate  26  and the pump cover  24  is provided to the side plate  26  disposed between the rotor  27  and the pump cover  24 . The intercommunication port  261  may be configured in such a size that the shaft hole  27 A and the space  80  intercommunicate with each other and the pressure difference between the shaft hole  27 A and the space  80  can be eliminated. In this embodiment, the intercommunication port  261  is configured to be smaller than the shaft diameter of the tip portion  12 A of the output shaft  12 . 
     According to this construction, the pressure difference between the shaft hole  27 A of the rotor  27  and the space  80  can be suppressed. Therefore, even when the side plate  26  is formed of a material having low rigidity such as carbon or the like, the side plate  26  can be prevented from slacking due to the pressure difference, and thus the contact between the rotor  27  and the side plate  26  can be prevented, whereby the abrasion of the rotor  27  and the side plate  26  can be suppressed and the durability of the vacuum pump  1  can be enhanced. 
     Here, the volume of the space  80  is extremely smaller than that of the cylinder chamber S. Therefore, even when the size of the intercommunication port  261  is smaller than the shaft diameter of the tip portion  12 A of the output shaft  12 , the pressure difference between the shaft hole  27 A of the rotor  27  and the space  80  can be rapidly eliminated. On the other hand, when the intercommunication port  261  is formed to be larger than the shaft diameter of the tip portion  12 A of the output shaft  12 , excessive air flows from the space  80  through the intercommunication port  261  into the cylinder chamber S, and thus it is assumed that the performance of the vacuum pump degrades due to reduction of the compressibility. 
     Accordingly, in this embodiment, the size of the intercommunication port  261  is set to be smaller than the shaft diameter of the tip portion  12 A of the output shaft  12 , whereby the pressure difference between the shaft hole  27  of the rotor  27  and the space  80  can be quickly eliminated, and the reduction of the compressibility when the rotor  27  is rotated can be prevented, so that the performance of the vacuum pump  1  can be prevented from being degraded. 
     As shown in  FIG. 5 , the intercommunication port  261  is formed on the axial center of the shaft hole  27 A of the rotor  27 , that is, on the rotational center X1. In  FIG. 5 , the side plate  26  is illustrated by a broken line for convenience of description. The rotor  27  rotates based on the rotational center X1 together with the output shaft  12 , and the rotational center X1 axis corresponds to the position which has the lowest influence on the compression and expansion when the rotor  27  is rotated. Accordingly, by forming the intercommunication port  261  on the axial center of the shaft hole  27 A of the rotor  27 , the reduction of the compressibility when the rotor  27  is rotated can be further prevented and the degradation of the performance of the vacuum pump  1  can be prevented while keeping the function of eliminating the pressure difference between the shaft hole  27 A of the rotor  27  and the space  80 . In this embodiment, the intercommunication port  261  is formed on the axial center of the shaft hole  27 A of the rotor  27 . However, the present invention is not limited to this construction, and the intercommunication port  261  may be disposed within an area which confronts the recess portion  27 H at the front end surface  27 G side of the rotor  27 . 
     Furthermore, in this embodiment, as shown in  FIG. 4 , the casing body  22  has the seal groove  22 G formed around the cylinder chamber S, and a seal member  81  through which the exhaust passage  37  for exhausting air from the cylinder chamber S to the outside of the machine and the space  80  are isolated from each other is disposed in the seal groove  22 G. Accordingly, the exhausted air is prevented from flowing into the space  80  by the seal member  81 , and the contact between the rotor  27  and the side plate  26  can be surely prevented. Furthermore, atmospheric pressure air can be prevented from flowing back into the cylinder chamber S, and thus the performance of the vacuum pump  1  can be prevented from degrading. 
     The best modes for carrying out the invention has been described. However, the present invention is not limited to the above embodiment, and various modifications and alterations can be made on the basis of the technical idea of the present invention. For example, in this embodiment, the female screw formed at the shaft hole  27 A of the rotor  27  and the nut  70  are engaged with the male screw provided to the tip portion  12 A of the output shaft  12  to fix the rotor  27 . However, the rotor  27  may be fixed by another fixing means. In this case, it is assumed that the recess portion  27 H is not formed at the front end surface  27 G of the rotor  27 . However, in this construction, the intercommunication port  261  may be formed within an area corresponding to the shaft hole  27 A. 
     Second Embodiment 
     A vacuum pump having a rotating and compressing element driven by an electric motor provided in a casing is generally known. This type of vacuum pump is used to generate vacuum for actuating a power braking device of a vehicle, for example, and vacuum can be obtained by driving the rotating and compressing element in a cylinder chamber provided to the casing. 
     This type of vacuum pump is configured so that the electric motor and the casing having the rotating and compressing element are connected to each other, and the rotating and compressing element connected to the rotating shaft of the electric motor slides in the cylinder chamber. Therefore, it is important to assemble the casing in conformity with the rotational center of the rotating shaft of the electric motor. 
     Accordingly, this applicant has proposed a vacuum pump in which a fitting bore portion having the rotational center of the rotating shaft at the center thereof is formed at one end side of the case of the electric motor, a cylindrical fitting portion protruding to the periphery of the cylinder chamber is formed on the back surface of the casing, and the fitting portion is faucet-fitted to the fitting bore portion of the electric motor, whereby the positioning can be accurately and easily performed under an assembling work (JP-A-2011-214519). 
     However, the above construction has a risk that when the electric motor and the casing are assembled with each other, the misalignment corresponding to the clearance of fitting tolerance between the fitting bore portion and the fitting portion occurs between the cylinder chamber and the rotating and compressing element, so that individual difference occurs in the performance of the vacuum pump. Furthermore, in this construction, the fitting bore portion is formed in the case of the electric motor, and the fitting portion is formed in the casing. Therefore, this construction has a problem that different molds are required to form these members, and thus the manufacturing cost increases. 
     Therefore, the present invention has been implemented in view of the foregoing situation, and has an object to provide a vacuum pump which can reduce the manufacturing cost, suppress misalignment occurring under the assembling work and exercise substantially uniform performance. 
     Next, a vacuum pump according to the second embodiment will be described. As in the case of the vacuum pump of the first embodiment, the vacuum pump according to the second embodiment is used for a braking device using the vacuum pump as a negative pressure source. Application of the vacuum pump according to the second embodiment is the same as the first embodiment described above, and the description thereof is omitted. 
       FIG. 6  is a partially sectional view of the side portion of a vacuum pump  101 , and  FIG. 7  is a view of the vacuum pump  101  when the vacuum pump  101  is viewed from the rear side. However,  FIG. 7  shows a state that members such as a pump cover  124 , a side plate  126 , etc. are detached to show the construction of the cylinder chamber S. In the following description, the directions represented by arrows at the upper portion of  FIGS. 6 and 7  represent upper, lower, front, rear, right and left sides of the vacuum pump  101  for convenience of description. The front-and-rear direction is also referred to as “axial direction”, and the right-and-left direction is also referred to as “width direction”. 
     As shown in  FIG. 6 , the vacuum pump  101  has an electrical motor  110 , and a pump body  120  operated by the electric motor  110  as a driving source. The electric motor  110  and the pump body  120  are fixed and supported in a vehicle body such as a car or the like while connected integrally with each other. 
     The electric motor  110  has an output shaft (rotating shaft)  112  extending from the substantially center portion of one end portion (rear end) of a substantially cylindrical motor case body  111  to the pump body  120  side (rear side). The output shaft  112  functions as a driving shaft for driving the pump body  120 , and rotates around the rotational center X1 extending in the front-and-rear direction. A male screw which is threadably fitted to a screw hole provided to the rotor  127  of the pump body  120  is formed at the tip portion  112 A of the output shaft  112 , and the output shaft  112  and the rotor  127  are connected to each other to be integrally rotatable. Furthermore, in this embodiment, a nut  170  is engaged with the male screw of the output shaft  112  at the tip side of the rotor  127 , thereby restricting movement of the rotor  127  to the tip side of the output shaft  112 . 
     When the electric motor  110  is powered by a power source (not shown), the output shaft  112  rotates in the direction of an arrow R (counterclockwise) in  FIG. 7 , whereby the rotor  127  is rotated in the same direction (the direction of the arrow R) around the rotational center X1. 
     The motor case body  111  is configured in a substantially cylindrical shape having a bottom to have an opening portion  111 A at one end thereof, the opening portion  111 A side thereof is fixed to the pump body  120 . Specifically, the motor case body  111  has a flange portion  111 B which is integrally formed by bending the peripheral edge of the opening portion  111 A outwards, and the flange portion  111 B is fixed to the pump body  120  by screws  160 . 
     As shown in  FIG. 6 , the pump body  120  has a casing body  122  secured to the flange portion  111 B formed at the rear side of the motor case body  111  of the electric motor  110 , a cylinder liner  123  which is press-fitted in the casing body  122  to form the cylinder chamber S, and a pump cover  124  which covers the casing body  122  from the rear side. In this embodiment, the casing  131  of the vacuum pump  101  is configured to have the casing body  122 , the cylinder liner  123  and the pump cover  124 . 
     The casing body  122  is formed of metal material having high thermal conductivity such as aluminum or the like, and configured in a substantially rectangular shape to be longer in the up-and-down direction with the rotational center X1 located substantially at the center when it is viewed from the rear side as shown in  FIG. 7 . An intercommunication hole  122 A which intercommunicates with the inside of the cylinder S provided to the casing body  122  is formed at one side surface (right side surface) portion of the casing body  122 , and a vacuum suction nipple  130  is press-fitted in the intercommunication hole  122 A. As shown in  FIG. 6 , the vacuum suction nipple  130  is a straight pipe extending outwards in the width direction, and a pipe or tube for supplying negative-pressure air from external equipment (for example, the vacuum tank  7  (see  FIG. 1 )) is connected to one end  130 A of the vacuum suction nipple  130 . 
     The casing body  122  has a bore portion  172  which extends from the rear end (open end) to some point of the front side based on the axial center X2 extending in the front-and-rear direction, and a cylindrical cylinder liner  123  is press-fitted in the bore portion  172 . It is needless to say that the cylinder liner  123  is not press-fitted in the bore portion  172 , but fitted in the bore portion  172 . 
     The axial center X2 is parallel to the rotational center X1 of the output shaft  112  of the electric motor  110 , and eccentrically displaced from the rotational center X1 to the upper right side as shown in  FIG. 6 . In this construction, the axial center X2 is eccentrically displaced so that the outer peripheral surface  127 B of the rotor  127  having the rotational center X1 at the center thereof makes contact with the inner peripheral surface  123 A of the cylinder liner  123  formed based on the axial center X2. 
     The cylinder liner  123  is formed of the same metal material (iron in this embodiment) as the rotor  127 . In this construction, the cylinder liner  123  and the rotor  127  have the same thermal expansion coefficient. Therefore, the contact between the outer peripheral surface  127 B of the rotor  127  and the inner peripheral surface  123 A of the cylinder liner  123  when the rotor  127  is rotated can be prevented irrespective of temperature variation of the cylinder liner  123  and the rotor  127 . When the cylinder liner  123  and the rotor  127  may be formed of different materials insofar as these materials have substantially the same level thermal expansion coefficients. 
     Since the cylinder liner  123  can be accommodated within the length range in the front-and-rear direction of the casing body  122  by press-fitting the cylinder liner  123  into the bore portion  172  formed in the casing body  122 , the cylinder liner  123  can be prevented from protruding from the casing body  122 , and the casing body  122  can be miniaturized. 
     Furthermore, the casing body  122  is formed of a material having higher thermal conductivity than the rotor  127 . Accordingly, heat occurring when the rotor  127  and the vanes  128  are rotationally driven can be quickly transferred to the casing body  122 , and thus heat can be sufficiently radiated from the casing body  122 . 
     An air intake port  123 B through which the intercommunication hole  122 A of the casing body  122  and the cylinder chamber S intercommunicate with each other is formed in the cylinder liner  123 , air passing through the vacuum suction nipple  130  is supplied through the intercommunication hole  122 A and the air intake port  123 B into the cylinder chamber S. Discharge ports  122 C,  123 C which penetrate through the casing body  122  and the cylinder liner  123  and through which air compressed in the cylinder chamber S is discharge are formed at the other side surface (left side surface) portion side of the casing body  122  in the casing body  122  and the cylinder liner  123 . The discharge ports  122 C,  123 C are formed on the same axis as the intercommunication hole  122 A and the air intake port  123 B. 
     Side plates  125 ,  126  which block the opening of the cylinder chamber S are disposed at the front end and rear end of the cylinder liner  123 , respectively. The diameters of these side plates  125 ,  126  are set to be larger than the inner diameter of the inner peripheral surface  123  of the cylinder linear  123 , and urged to be pressed against the front end and rear end of the cylinder liner  123  by seal rings  125 A,  126 A, respectively. Accordingly, the cylinder chamber S which is hermetically closed except for the air intake port  123 B intercommunicating with the vacuum suction nipple  130  and the discharge ports  123 C,  122 C is formed inside the cylinder liner  123 . 
     In this embodiment, the side plate  126  at the electric motor  110  side is disposed at the terminal of the bore portion  172 , and pinched through a sealing ring  126 A between the wall portion  172 A of the bore portion  172  and the cylinder liner  123 . 
     The rotor  127  is disposed in the cylinder chamber S. The rotor  127  has a circular cylindrical shape extending along the rotational center X1 of the electric motor  110 , and has a shaft hole  127 A to which the output shaft  112  as the driving shaft of the pump body  120  is threadably fitted. In addition, plural guide grooves  127 C are provided to be far away radially from the shaft hole  127 A and spaced from one another at equiangular intervals in the peripheral direction around the shaft hole  127 A. As shown in  FIG. 6 , a recess portion  127 H is formed at the end face (so-called rear end face)  127 G at the side of the rotor  127  which confronts the pump cover  124 , and the nut  70  is threadably fitted to the male screw of the output shaft  112  in the recess portion  127 H. In this embodiment, the length of the shaft end of the output shaft  112  extending in the recess portion  127 H and the thickness of the nut  170  are set to be substantially equal to or slightly smaller than the depth of the recess portion  127 H respectively, so that the output shaft  112  and the nut  170  are prevented from protruding from the rear end face  127 G of the rotor  127 . 
     The length in the front-and-rear direction of the rotor  127  is set to be substantially equal to the length of the cylinder chamber S of the cylinder liner  123 , that is, the distance between the confronting inner surfaces of the two side plates  125 ,  126 , and the gap between the rotor  127  and the side plates  125 ,  126  is substantially closed. 
     The outer diameter of the rotor  127  is set so that the outer peripheral surface  127 B of the rotor  127  keeps a minute clearance from a portion located at a lower left side out of the inner peripheral surface  123 A of the cylinder liner  123  as shown in  FIG. 7 . Accordingly, as shown in  FIG. 7 , a crescent-shaped space is formed between the outer peripheral surface  127 B of the rotor  127  and the inner peripheral surface  123 A of the cylinder liner  123 . 
     Plural (five in this embodiment) vanes  128  for sectioning the crescent-shaped space are provided to the rotor  127 . The vane  128  is configured like a plate, and the length thereof in the front-and-rear direction is set to be substantially equal to the distance between the mutually confronting inner surfaces of the two side plates  125 ,  126  as in the case of the rotor  127 . These vanes  128  are disposed to freely protrude from and retract into the guide grooves  127 C provided to the rotor  127 . Each vane  128  protrudes outwards along the guide groove by centrifugal force thereof in connection with the rotation of the rotor  127 , and the tip thereof abuts against the inner peripheral surface  123 A of the cylinder liner  123 . Accordingly, the crescent-shaped space is sectioned into five compression chambers P surrounded by the respective adjacent two vanes  128 ,  128 , the outer peripheral surface  127 B of the rotor  127  and the inner peripheral surface  123 A of the cylinder liner  123 . In connection with the rotation of the rotor  127  in the direction of the arrow R which is caused by the rotation of the output shaft  112 , these compression chambers P rotate in the same direction, and the volume thereof increases in the neighborhood of the air intake port  123 B while the volume thereof decreases at the discharge port  123 C. That is, through the rotation of the rotor  127  and the vanes  128 , air sucked from the air intake port  123 B into one compression chamber P is compressed and discharged from the discharge port  123 C while going around in connection with the rotation of the rotor  127 . 
     An exhaust portion  132  is secured to the left side surface of the casing body  122  having the discharge port  122 C formed therein so as to surround the discharge port  122 C. The exhaust portion  132  has an expansion portion  132 A which expands outwards in the width direction substantially at the center thereof, and a peripheral edge portion  132 B which is provided around the expansion portion  132 A and comes in close contact with the left side surface of the casing body  122 , and the peripheral edge portion  132 B is secured to the casing body  122  by screws  164 . An exhaust port  132 C through which air discharged from the discharge port  123 C is discharged to the outside of the machine (the outside of the vacuum pump  101 ) is provided to the expansion portion  132 A, and a check valve  129  is secured to the exhaust port  132 C to prevent flowback of the air from the outside of the machine to the pump. 
     The pump cover  124  is disposed on the side plate  126  at the front side through a seal ring  126 A, and fixed to the casing body  122  by bolts  166 . As shown in  FIG. 6 , a seal groove  122 D is formed on the rear end face of the casing body  122  so as to surround the cylinder liner  123 , and an annular seal member  167  is disposed in the seal groove  122 D. 
     As described above, the vacuum pump  101  is constructed by connecting the electric motor  110  and the pump body  120 , and the rotor  127  connected to the output shaft  112  of the electric motor  110  and the vanes  128  slide in the cylinder liner  123  of the pump body  120 . Therefore, it is important to assemble the pump body  120  in conformity with the rotational center X1 of the output shaft  112  of the electric motor  110 . 
     In this embodiment, a through hole  173  through which the output shaft  112  penetrates, and an annular bearing holding portion  174  provided around the through hole  173  are formed substantially at the center of a face of the casing body  122  to which the electric motor  110  is secured, and the outer ring of a bearing (bearing portion)  175  for supporting the output shaft  112  is held on the inner peripheral surface  174 A of the bearing holding portion  174 . The through hole  173  and the bearing holding portion  174  are formed so that the rotational center X1 is set at the center thereof, and formed in the casing body  122  integrally with the bore portion  172  in which the cylinder liner  123  is press-fitted. Accordingly, when the bore portion  172  and the bearing holding portion  174  of the casing body  122  are provided with the cylinder liner  123  and the bearing  175  respectively, the positional relationship between the bearing  175  based on the rotational axis X1 and the cylinder liner  123  based on the axial center X2 can be regulated in the casing body  122 . Therefore, a misalignment occurring when the motor case body  111  of the electric motor  110  is assembled with the casing body  122  can be suppressed, and the assembled vacuum pump  101  can exercise substantially uniform performance having little individual difference. 
     Furthermore, the casing body  122  can be formed by using a single mold, so that the number of parts can be reduced and thus the manufacturing cost can be reduced. 
       FIG. 8  is a partially enlarged view of  FIG. 6 . 
     As described above, the cylinder liner  123  is press-fitted in the bore portion  172  formed in the casing body  122 . In this construction, the bore portion  172  is formed as a stepped bore which decreases in diameter from the rear end (open end) of the casing body  122  to the depth side (wall portion  72 A) of the casing body  122 , and has a liner holding portion  172 B in which the cylinder liner  123  is held, a diameter-reduced portion  172 C which is smaller in diameter than the liner holding portion  172 B and in which the side plate  126  is disposed, and a step portion  172 D formed between the liner holding portion  172 B and the diameter-reduced portion  172 C. 
     Accordingly, the press-fitting work of the cylinder liner can be easily and accurately performed by press-fitting the cylinder liner  123  so that the cylinder liner  123  abuts against the step portion  172 D. 
     Furthermore, the bore diameter of the diameter-reduced portion  172 C is set to be larger than the inner diameter of the cylinder liner  123 , and thus the side plate  126  which is larger than the inner diameter of the cylinder liner  123  can be disposed at the diameter-reduced portion  72 C, so that the opening of the cylinder liner  123  can be simply blocked by the side plate  126 . 
     The best modes for carrying out the present invention have been described. However, the present invention is not limited to the above embodiments, and various modifications and alterations can be made on the basis of the technical idea of the present invention. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
         
           
               1  vacuum pump 
               6  brake booster (power braking device) 
               7  vacuum tank 
               9  brake pipe 
               10  electric motor (driving machine) 
               11  case 
               12  output shaft (rotating shaft) 
               12 A tip portion 
               22  casing body 
               22 G seal groove 
               23  cylinder portion 
               25  side plate 
               26  side plate 
               27  rotor 
               27 A shaft hole 
               27 D shaft holding portion 
               27  rotor 
               27 A shaft hole 
               27 E shaft holding portion 
               27 F hole portion 
               27 G front end face 
               27 H recess portion 
               28  vane 
               70  nut 
               80  space (space between side plate and pump cover) 
               81  seal member 
               100  brake device 
               261  intercommunication port 
               101  vacuum pump 
               110  electric motor (motor) 
               111  motor case 
               111 A opening portion 
               112  output shaft (rotating shaft) 
               122  casing body 
               123  cylinder liner 
               127  rotor (rotating and compressing element) 
               128  vane (rotating and compressing element) 
               131  casing 
               172  bore portion 
               172 C diameter-reduced portion 
               174  bearing holding portion 
               175  bearing (bearing portion)