ROOTS PUMP

BACKGROUND OF THE INVENTION

The present invention relates to a roots pump, which traps fluid, which is drawn into a suction chamber from an inlet, in a pressure chamber defined by an inner surface of a housing and a rotor, discharges the fluid trapped in the pressure chamber to a discharge chamber formed in the housing, and discharges the fluid from an outlet formed in the housing.

Japanese Laid-Open Patent Publication No. 2004-278350 discloses a roots fluid machine as one example of the roots pump. The roots type fluid machine of Japanese Laid-Open Patent Publication No. 2004-278350 is equipped with two three-lobe rotors each including three lobes and three recesses. The rotors have a shape extending straight along the axial direction of the rotary shafts. In the roots type fluid machine that uses such straight rotors, fluctuation of the volumetric change in the suction chamber (suction amount of fluid to the suction chamber per unit time) and fluctuation of the volumetric change in the discharge chamber (discharge amount of fluid from the discharge chamber per unit time) are great, and thus the suction pulsation and the discharge pulsation are great.

Japanese Laid-Open Utility Model Publication No. 62-71392 discloses a mechanical supercharger as one example of the roots pump. In a mechanical supercharger 80 of Japanese Laid-Open Utility Model Publication No. 62-71392, two rotor shafts 82, 83 are rotatably supported by a casing (housing) 81 as shown in FIG. 9, and first and second inlets and first and second outlets (both are not shown) are formed. Also, rotors 86, 87 are mounted on one end portion of the rotor shafts 82, 83. The rotors 86, 87 are rotated by the rotation of the rotor shafts 82, 83 and operate together to discharge fluid from the first inlet to the first outlet. The pair of rotors 86, 87 configure a first rotor pair R1. Furthermore, rotors 88, 89 are mounted on the other end portion of the rotor shafts 82, 83. The rotors 88, 89 are rotated by the rotation of the rotor shafts 82, 83, and operate together to discharge fluid from the second inlet to the second outlet. The pair of rotors 88, 89 configure a second rotor pair R2. That is, the mechanical supercharger 80 is a parallel roots pump including two rotor pairs R1, R2.

Also, the casing 81 is provided with a partition wall 90, which divides the casing 81 into a section corresponding to the first rotor pair R1 and a section corresponding to the second rotor pair R2. The first inlet and the first outlet are formed in the casing 81 at positions corresponding to the first rotor pair R1, and the second inlet and the second outlet are formed in the casing 81 at positions corresponding to the second rotor pair R2. Among the first rotor pair R1 and the second rotor pair R2, the rotors located on the same one of the rotor shafts 82, 83 (the rotor 86 and the rotor 88, the rotor 87 and the rotor 89) have different mounting phase angles from each other with respect to the associated one of the rotor shafts 82, 83. Therefore, in the mechanical supercharger 80, the discharge pulsation of the first rotor pair R1 and the discharge pulsation of the second rotor pair R2 cancel each other. That is, the pulsation of the mechanical supercharger of Japanese Laid-Open Utility Model Publication No. 62-71392 is less than the pulsation of the roots type fluid machine of Japanese Laid-Open Patent Publication No. 2004-278350.

However, in the parallel mechanical supercharger (roots pump) 80 equipped with the rotor pairs R1, R2, a clearance is provided between the end surfaces of the rotors 86, 87 of the first rotor pair R1 and the partition wall 90, and between the end surfaces of the rotors 88, 89 of the second rotor pair R2 and the partition wall 90. Thus, the fluid trapped in the clearance (pressure chamber) between the rotors 86, 87 of the first rotor pair R1 and the inner surface of the casing 81, and fluid trapped in the clearance (pressure chamber) between the rotors 88, 89 of the second rotor pair R2 and the inner surface of the casing 81 leak from the clearances, thus reducing the transfer efficiency of fluid.

SUMMARY OF THE INVENTION

Accordingly, it is an objective of the present invention to provide a roots pump that reduces pulsation, and has an improved transfer efficiency of fluid compared to a parallel roots pump.

To achieve the above objective, and in accordance with a first aspect of the present invention, a roots pump including a housing, a pair of rotary shafts, and a pair of rotors is provided. The housing includes an inlet, a suction chamber, a discharge chamber, and an outlet. The pair of rotary shafts are rotatably supported by the housing. The pair of rotors are multi-lobe rotors. Each of the pair of rotors is provided on one of the pair of rotary shafts. The pair of rotors are accommodated in the housing. An inner surface of the housing and the pair of rotors define a pressure chamber. As the pair of rotors are rotated, fluid drawn into the suction chamber from the inlet is trapped in the pressure chamber, discharged to the discharge chamber from the pressure chamber, and then discharged through the outlet. Each of the pair of rotors includes a first straight portion and a second straight portion. The first straight portion is provided on one of end portions of the rotary shaft in an axial direction to extend straight along the axial direction. The second straight portion is provided on the other one of the end portions to extend straight along the axial direction. The phases of the first straight portion and the second straight portion are displaced along the circumferential direction of the associated rotary shaft. Each of the pair of rotors further includes a coupling portion, which couples the first straight portion and the second straight portion. The coupling portions of the pair of rotors are engaged with each other to suppress fluid leakage between the pair of rotors.

In accordance with a second aspect of the present invention, a roots pump including a housing, a pair of rotary shaft, and a pair of rotors is provided. The housing includes an inlet, a suction chamber, a discharge chamber, and an outlet. The pair of rotary shafts are rotatably supported by the housing. The pair of rotary shafts are rotated in opposite directions to each other. The pair of rotors are multi-lobe rotors. Each of the pair of rotors is provided on one of the pair of rotary shafts. The pair of rotors are accommodated in the housing. An inner surface of the housing and the pair of rotors define a pressure chamber. As the pair of rotors are rotated in opposite directions to each other, fluid drawn into the suction chamber from the inlet is trapped in the pressure chamber, discharged to the discharge chamber from the pressure chamber, and then discharged through the outlet. The pair of rotors each include at least a first straight portion and a second straight portion. The first straight portion and the second straight portion extend straight along the axial direction of the associated rotary shaft. The phases of the first straight portion and the second straight portion are displaced along the circumferential direction of the associated rotary shaft. The direction of the phase displacement of the second straight portion with respect to the first straight portion is opposite between the pair of rotors. Each of the pair of rotors further includes a coupling portion, which couples the first straight portion and the second straight portion. The coupling portions of the pair of rotors are engaged with each other to suppress fluid leakage between the pair of rotors.

Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention that are believed to be novel are set forth with particularity in the appended claims. The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:

FIG. 1 is a cross-sectional plan view illustrating a roots pump according to a first embodiment of the present invention;

FIG. 2 is a cross-sectional view taken along line 2-2 in FIG. 1;

FIG. 3 is a cross-sectional view taken along line 3-3 in FIG. 1;

FIG. 4 is a perspective view illustrating the rotor provided in the pump of FIG. 1;

FIG. 5 is a cross-sectional view illustrating a first straight portion of a roots pump according to a second embodiment of the present invention;

FIG. 6 is a cross-sectional view illustrating a second straight portion of the roots pump according to the second embodiment of the present invention;

FIG. 7 is a perspective view illustrating the rotors provided in the pump of FIG. 5;

FIG. 8A and FIG. 8B are diagrams for explaining clearances between the first rotor and the second rotor; and

FIG. 9 is a cross-sectional view illustrating a mechanical supercharger of the background art.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A roots pump 11 according to a first embodiment of the present invention will now be described with reference to FIGS. 1 to 4. In the following explanations, the front and rear of the roots pump 11 correspond to arrow Y in FIG. 1.

As shown in FIG. 1, a housing (pump housing) of the roots pump 11 includes a rotor housing member 12, a shaft support member 13, which is secured to the rear end of the rotor housing member 12, a gear housing member 14, which is secured to the rear end of the shaft support member 13, and a motor housing member 16, which is secured to the front end of the rotor housing member 12 with a partition 15 in between. The rotor housing member 12 and the shaft support member 13 define a pump chamber 17. As for the pump chamber 17, the inner surface of the rotor housing member 12 and the inner surface of the shaft support member 13 form an inner surface of the housing, which is an inner surface H of the pump chamber 17.

The gear housing member 14 and the shaft support member 13 define a gear chamber 18. The partition 15 and the motor housing member 16 define a motor chamber 19, which accommodates an unillustrated electric motor.

The motor housing member 16, the rotor housing member 12, and the shaft support member 13 rotatably support a rotary shaft, which is a first rotary shaft 21 in the first embodiment, with bearings 22. Furthermore, the rotor housing member 12 and the shaft support member 13 rotatably support a rotary shaft, which is a second rotary shaft 23 in the first embodiment, with bearings 24. The second rotary shaft 23 is parallel to the first rotary shaft 21.

In the gear chamber 18, a drive gear 27, which is secured to the first rotary shaft 21, and a driven gear 28, which is secured to the second rotary shaft 23, mesh with each other.

As shown in FIGS. 1 to 3, in the pump chamber 17, a rotor, which is a first rotor 30 in the first embodiment, is mounted on the first rotary shaft 21. The first rotor 30 rotates integrally with the first rotary shaft 21. A rotor, which is a second rotor 35 in the first embodiment, is mounted on the second rotary shaft 23. The second rotor 35 rotates integrally with the second rotary shaft 23. As shown in FIG. 2, the first rotor 30 is a three-lobe (multiple-lobe) rotor the cross section of which in a direction perpendicular to the axial direction of the first rotary shaft 21 has a three-lobe shape, and the second rotor 35 is a three-lobe (multiple-lobe) rotor the cross section of which in a direction perpendicular to the axial direction of the second rotary shaft 23 has a three-lobe shape.

As shown in FIG. 2, an inlet 12a for drawing fluid into the pump chamber 17 is formed in the rotor housing member 12, which configures part of the pump housing. Also, an outlet 12b for discharging the fluid in the pump chamber 17 outside of the pump chamber 17 is formed in the rotor housing member 12 at a position opposite to the inlet 12a.

The first rotor 30 and the second rotor 35 will be described in detail.

As shown in FIGS. 2 and 3, three lobes 31 are formed on the rotor 30, and a recess 32 is formed between each adjacent pair of the lobes 31. The lobes 31 and the recesses 32 located on a first end portion (front end portion) of the rotor 30 in the axial direction of the rotary shaft 21 extend straight along the axial direction of the rotary shaft 21. The lobes 31 and the recesses 32 located on the first end portion (front side portion) along the axial direction of the rotary shaft 21 form a first straight portion 30a of the rotor 30.

Similarly, three lobes 36 are formed on the rotor 35, and a recess 37 is formed between each adjacent pair of the lobes 36. The lobes 36 and the recesses 37 located on a first end portion (front end portion) of the rotor 35 in the axial direction of the rotary shaft 23 extend straight along the axial direction of the rotary shaft 23. The lobes 36 and the recesses 37 located on the first end portion (front end portion) in the axial direction of the rotary shaft 23 form a first straight portion 35a of the rotor 35.

Also, the lobes 31 and the recesses 32 located on a second end portion (rear end portion) of the rotor 30 in the axial direction of the rotary shaft 21 extend straight along the axial direction of the rotary shaft 21. The lobes 31 and the recesses 32 located at the second end portion (front end portion) along the axial direction of the rotary shaft 21 form a second straight portion 30b of the rotor 30.

Similarly, the lobes 36 and the recesses 37 located at a second end portion (rear end portion) of the rotor 35 in the axial direction of the rotary shaft 23 extend straight along the axial direction of the rotary shaft 23. The lobes 36 and the recesses 37 located at the second end portion (front end portion) in the axial direction of the rotary shaft 23 form a second straight portion 35b of the rotor 35.

Furthermore, the lobes 31 and the recesses 32 of the rotor 30 located between the first straight portion 30a and the second straight portion 30b form a coupling portion 30c, which connects the first straight portion 30a and the second straight portion 30b. The first rotor 30 integrally includes the first straight portion 30a, the second straight portion 30b, and the coupling portion 30c.

Similarly, the lobes 36 and the recesses 37 of the rotor 35 located between the first straight portion 35a and the second straight portion 35b form a coupling portion 35c, which connects the first straight portion 35a and the second straight portion 35b. The second rotor 35 integrally includes the first straight portion 35a, the second straight portion 35b, and the coupling portion 35c.

As shown in FIG. 1, the inlet 12a is formed in the rotor housing member 12 to face a position where the coupling portion 30c of the first rotor 30 and the coupling portion 35c of the second rotor 35 engage with each other. The length of the inlet 12a in the axial direction of the rotary shafts 21, 23 is greater than or equal to the length of the coupling portions 30c, 35c in the axial direction of the rotary shafts 21, 23. Also, the outlet 12b is formed in the rotor housing member 12 to face a position where the coupling portion 30c of the first rotor 30 and the coupling portion 35c of the second rotor 35 engage with each other. The length of the outlet 12b in the axial direction of the rotary shafts 21, 23 is greater than or equal to the length of the coupling portion 35c in the axial direction of the rotary shafts 21, 23.

As shown in FIG. 1, the length L1 of the first straight portion 30a of the rotor 30 in the axial direction of the rotary shaft 21 is equal to the length L2 of the second straight portion 30b of the rotor 30 in the axial direction of the rotary shaft 21. Similarly, the length of the first straight portion 35a of the rotor 35 in the axial direction of the rotary shaft 23 and the length of the second straight portion 35b of the rotor 35 in the axial direction of the rotary shaft 23 are the same length L1 (=L2). The coupling portion 30c is located at the center portion of the length of the first rotor 30 along the axial direction of the rotary shaft 21. In the first embodiment, the length of the coupling portions 30c, 35c is less than L1, L2.

A slight clearance CL1 is formed between an end surface 301 of the first straight portion 30a and the inner surface H of the pump chamber 17 (the inner surface of the rotor housing member 12). The end surface 301 extends in a direction perpendicular to the axial direction of the rotary shaft 21. Furthermore, a slight clearance CL2 is formed between an end surface 302 of the second straight portion 30b and the inner surface H of the pump chamber 17 (the end surface of the shaft support member 13). The end surface 302 extends in a direction perpendicular to the axial direction of the rotary shaft 21.

Similarly, a slight clearance CL3 is formed between an end surface 351 of the first straight portion 35a and the inner surface H of the pump chamber 17 (the inner surface of the rotor housing member 12). The end surface 351 extends in a direction perpendicular to the axial direction of the rotary shaft 23. Furthermore, a slight clearance CL4 is formed between an end surface 352 of the second straight portion 35b and the inner surface H of the pump chamber 17 (the end surface of the shaft support member 13). The end surface 352 extends perpendicular to the axial direction of the rotary shaft 23.

The clearances CL1, CL2, CL3, and CL4 prevent the end surfaces 301, 351 of the first straight portions 30a, 35a of the rotors 30, 35 from sliding on the inner surface H of the pump chamber 17, and the end surfaces 302, 352 of the second straight portions 30b, 35b from sliding on the inner surface H of the pump chamber 17 to prevent, for example, sticking. The clearances CL1, CL2, CL3, and CL4 are small gaps to minimize leakage of fluid.

As shown in FIG. 2, as for the first straight portion 30a of the first rotor 30, assuming that straight lines that connect tips Ta of the lobes 31 and the central axis P1 of the rotary shaft 21 (the rotation axis of the first rotor 30) are imaginary lines M1, M2, and M3, respectively, the angle between each adjacent pair of the imaginary lines (the imaginary line M1 and the imaginary line M2, the imaginary line M2 and the imaginary line M3, and the imaginary line M3 and the imaginary line Ml) is 120°. Similarly, as for the first straight portion 35a of the second rotor 35, assuming that straight lines that connect tips Tc of the lobes 36 and the central axis P2 of the rotary shaft 23 (rotation axis of the second rotor 35) are imaginary lines M4, M5, and M6, respectively, the angle between each adjacent pair of the imaginary lines (the imaginary line M4 and the imaginary line M5, the imaginary line M5 and the imaginary line M6, and the imaginary line M6 and the imaginary line M4) is 120°.

Furthermore, as for the second straight portion 30b of the first rotor 30, assuming that straight lines, which connect tips Tb of the lobes 31 and the central axis P1 of the rotary shaft 21, are imaginary lines N1, N2, and N3, the angle between each adjacent pair of the imaginary lines (the imaginary line N1 and the imaginary line N2, the imaginary line N2 and the imaginary line N3, and the imaginary line N3 and the imaginary line N1) is 120°. Similarly, as for the second straight portion 35b of the second rotor 35, assuming that straight lines that connect tips Td of the lobes 36 and the central axis P2 of the rotary shaft 23 are imaginary lines N4, N5, and N6, respectively, the angle between each adjacent pair of the imaginary lines (the imaginary line N4 and the imaginary line N5, the imaginary line N5 and the imaginary line N6, and the imaginary line N6 and the imaginary line N4) is 120°.

In the first rotor 30, the angle θ1 between each of the imaginary lines M1 to M3 of the lobes 31 of the first straight portion 30a and the adjacent and closer one of the imaginary lines N1 to N3 of the lobes 31 of the second straight portion 30b that are displaced in the circumferential direction of the rotary shaft 21 as viewed from the front is 30°. That is, the first straight portion 30a and the second straight portion 30b are located at positions the phases of which are displaced in the circumferential direction of the rotary shaft 21. As shown in FIG. 4, when the first rotor 30 is viewed from the front side, the phase of the second straight portion 30b is displaced counterclockwise with respect to the first straight portion 30a about the central axis P1 by 30°. Hereinafter, “to displace the phase counterclockwise about the central axis P1 by 30° as viewed from the front side of the first rotor 30” is expressed as “counterclockwise phase displacement of 30°”.

As shown in FIG. 2, in the second rotor 35, the angle θ2 between each of the imaginary lines M4 to M6 of the lobes 36 of the first straight portion 35a and adjacent and closer one of the imaginary lines N4 to N6 of the lobes 36 of the second straight portion 35b that are displaced in the circumferential direction of the rotary shaft 23 as viewed from the front is 30°. That is, the first straight portion 35a and the second straight portion 35b are located at positions the phases of which are displaced in the circumferential direction of the rotary shaft 23. As shown in FIG. 4, when the second rotor 35 is viewed from the front side, the phase of the second straight portion 35b is displaced clockwise with respect to the first straight portion 35a about the central axis P2 by 30°. Hereinafter, “to displace the phase clockwise about the central axis P2 by 300 as viewed from the front side of the second rotor 35” is expressed as “clockwise phase displacement of 30°”.

As shown in FIG. 4, the lobes 31 and the recesses 32 of the coupling portion 30c of the first rotor 30 extend helically (counterclockwise) along the circumferential direction of the rotary shaft 21 from one to the other of the first straight portion 30a and the second straight portion 30b. The cross-sectional view of the coupling portion 30c in a direction perpendicular to the axial direction of the rotary shaft 21 has a three-lobe shape, as do the straight portions 30a, 30b, at any position in the axial direction of the rotary shaft 21.

Similarly, the lobes 36 and the recesses 37 of the coupling portion 35c of the second rotor 35 extend helically (clockwise) along the circumferential direction of the rotary shaft 23 from one to the other of the first straight portion 35a and the second straight portion 35b. The cross-sectional view of the coupling portion 35c in a direction perpendicular to the axial direction of the rotary shaft 23 has three-lobe shape, as do the straight portions 35a, 35b, at any position in the axial direction of the rotary shaft 23.

The lobes 31, 36 and the recesses 32, 37 of the first straight portions 30a, 35a of the first rotor 30 and the second rotor 35 engage one another. Also, the lobes 31, 36 and the recesses 32, 37 of the second straight portions 30b, 35b of the first rotor 30 and the second rotor 35 engage one another. Furthermore, the lobes 31, 36 and the recesses 32, 37 of the coupling portions 30c, 35c of the first rotor 30 and the second rotor 35 engage one another.

As shown in FIGS. 2 and 3, a pressure chamber D is defined between two adjacent lobes 31, 36 of the first straight portions 30a, 35a and the inner surface H of the pump chamber 17, between the coupling portions 30c, 35c and the inner surface H of the pump chamber 17, and between two adjacent lobes 31, 36 of the second straight portions 30b, 35b and the inner surface H of the pump chamber 17. The pressure chamber D traps fluid. Also, a space communicating with the inlet 12a and surrounded by the first rotor 30, the second rotor 35, and the inner surface H of the pump chamber 17 forms a suction chamber 40. Furthermore, a space communicating with the outlet 12b and surrounded by the first rotor 30, the second rotor 35, and the inner surface H of the pump chamber 17 forms a discharge chamber 41.

In the roots pump 11, when the first rotary shaft 21 is rotated by the electric motor, the second rotary shaft 23 is rotated in a direction different from the first rotary shaft 21 through the meshing engagement between the drive gear 27 and the driven gear 28. Then, the first rotor 30 and the second rotor 35 are rotated in directions opposite to each other, and fluid is drawn into the suction chamber 40 through the inlet 12a. The fluid in the suction chamber 40 is then trapped in the pressure chamber D as the rotors 30, 35 are rotated, and is transferred to the outlet 12b.

The first embodiment has the following advantages.

(1) In the roots pump equipped with the three-lobe rotors disclosed in Japanese Laid-Open Patent Publication No. 2004-278350, the volumetric change of the suction chamber fluctuates greatly every time the rotors are rotated by 60°, and fluid leakage between the pair of rotors fluctuates at the same timing as the great fluctuation of the volumetric change. Thus, the suction pulsation with the fundamental component of sixth order is generated. Also, since the volumetric change of the discharge chamber fluctuates greatly every time the rotors are rotated 60°, the discharge pulsation with the fundamental component of sixth order is generated. The volumetric change of the suction chamber refers to the change in the volume of the suction chamber per unit time (suction amount of fluid to the suction chamber per unit time) . The volumetric change of the discharge chamber refers to the change in the volume of the discharge chamber per unit time (discharge amount of fluid from the discharge chamber per unit time).

According to the first embodiment, in the first rotor 30, the phase of the second straight portion 30b is displaced counterclockwise with respect to the first straight portion 30a by 30°, and in the second rotor 35, the phase of the second straight portion 35b is displaced clockwise with respect to the first straight portion 35a by 30°. Therefore, the fluctuation of the volumetric change in the suction chamber 40 formed between the first rotor 30 and the second rotor 35, which rotate in the opposite directions to each other, is reduced as compared to the roots pump disclosed in Japanese Laid-Open Patent Publication No. 2004-278350. That is, the suction pulsation is suppressed in the roots pump 11 of the first embodiment.

Also, the fluctuation of the volumetric change in the discharge chamber 41 formed between the first rotor 30 and the second rotor 35, which rotate in the opposite directions to each other, is reduced as compared to the roots pump disclosed in Japanese Laid-Open Patent Publication No. 2004-278350. That is, the discharge pulsation is suppressed in the roots pump 11 of the first embodiment like the parallel roots pump disclosed in Japanese Laid-Open Utility Model Publication No. 62-71392.

On the assumption that the number of the lobes of the rotors 30, 35 (the number of lobes) is n, and the number of the straight portions 35a, 35b is X, when the angle θ(=θ1, θ2) of the phase displacement of the second straight portion 35b with respect to the first straight portion 35a satisfies the following expression (1), the pulsation is reduced.

θ=(360°/2n)/X . . . (1)

(2) In the first embodiment, the partition wall in the parallel roots pump disclosed in Japanese Laid-Open Utility Model Publication No. 62-71392 does not exist. Therefore, fluid leakage between the partition wall and the rotors as in the parallel roots pump of Japanese Laid-Open Utility Model Publication No. 62-71392 is not caused. Thus, the transfer efficiency of fluid is improved in the roots pump 11 with the rotors 30, 35, in which the first straight portion 30a is coupled to the second straight portion 30b by the coupling portion 30c and the first straight portion 35a is coupled to the second straight portion 35b by the coupling portion 35c, as compared to the parallel roots pump, which includes the partition wall in the housing.

(3) According to the parallel pump of Japanese Laid-Open Utility Model Publication No. 62-71392, the clearances between the partition wall and the rotors need to be adjusted accurately. In the first embodiment, since the partition wall of the parallel roots pump of Japanese Laid-Open Utility Model Publication No. 62-71392 does not exist, highly accurate clearance adjustment required in the pump of Japanese Laid-Open Utility Model Publication No. 62-71392 is unnecessary, and the manufacturing costs of the roots pump 11 is suppressed.

(4) The roots pump 11 including the first rotor 30 and the second rotor 35 reduces pulsation as compared to the roots pump in which the rotors extend straight along the entire axial direction of the rotary shafts (conventional roots pump). Thus, even if the rotation speed of the rotary shafts 21, 23 is increased, pulsation is not increased as compared to the conventional roots pump. As a result, the size of the roots pump 11 is reduced.

(5) In the first rotor 30 and the second rotor 35, the lengths of the rotary shafts 21, 23 along the axial direction at the first straight portions 30a, 35a and the second straight portions 30b, 35b are the same. Also, the coupling portions 30c, 35c are located at the center portion of the rotors 30, 35. Therefore, in the suction chamber 40 and the discharge chamber 41, the volumetric change at the first straight portions 30a, 35a and the volumetric change at the second straight portions 30b, 35b that occur while the rotary shafts 21, 23 are rotated once are equalized. When the axial length of the first straight portions 30a, 35a differ from the axial length of the second straight portions 30b, 35b, the volumetric change that occurs at the straight portion with a longer axial length is greater than the volumetric change that occurs at the straight portion with a shorter axial length. This increases the pulsation as compared to a case where the lengths are the same. Therefore, the axial lengths of the rotary shafts 21, 23 at the first straight portions 30a, 35a and the second straight portions 30b, 35b are preferably the same.

(6) The first rotor 30 and the second rotor 35 are three-lobe type including the three lobes 31, 36 and the three recesses 32, 37. The lobes 31, 36 at the first straight portions 30a, 35a and the lobes 31, 36 at the second straight portions 30b, 35B are provided at equal intervals along the circumferential direction of the rotary shafts 21, 23. Therefore, the intervals of the volumetric changes in the suction chamber 40 and the discharge chamber 41 that occur while the rotary shafts 21, 23 are rotated once are equal. Thus, the pulsation that occurs while the rotary shafts 21, 23 are rotated once takes place at equal intervals.

(7) The configuration in which the inlet 12a is provided to face the position where the coupling portion 30c of the first rotor 30 and the coupling portion 35c of the second rotor 35 engage with each other is preferable in reducing the suction pulsation since the distance from the first straight portions 30a, 35a and the distance from the second straight portions 30b, 35b are equal. Also, the configuration in which the outlet 12b is provided to face the position where the coupling portion 30c of the first rotor 30 and the coupling portion 35c of the second rotor 35 engage with each other is preferable in reducing the discharge pulsation since the distance from the first straight portions 30a, 35a and the distance from the second straight portions 30b, 35b are equal.

A second embodiment illustrated in FIGS. 5 to 8B will now be described. The same reference numerals are given to those components that are the same as the corresponding components of the first embodiment, and detailed explanations are omitted.

As shown in FIGS. 5 and 6, in the first rotor 30, the imaginary lines M1 to M3 extend from the tips Ta of the lobes 31 of the first straight portion 30a to the central axis P1, and the imaginary lines N1 to N3 extend from the tips Tb of the lobes 31 of the second straight portion 30b to the central axis P1. The angle θ1 between each of the imaginary lines M1 to M3 and the left adjacent one of the imaginary lines N1 to N3 is 60°. As shown in FIG. 7, when the first rotor 30 is viewed from the front side, the phase of the second straight portion 30b is displaced counterclockwise with respect to the first straight portion 30a about the central axis P1 by 60°. Hereinafter, “to displace the phase counterclockwise about the central axis P1 by 60° as viewed from the front side of the first rotor 30” is expressed as “counterclockwise phase displacement of 60°”.

As shown in FIGS. 5 and 6, in the second rotor 35, the imaginary lines M4 to M6 extend from the tips Tc of the lobes 36 of the first straight portion 35a to the central axis P2, and the imaginary lines N4 to N6 extend from the tips Td of the lobes 36 of the second straight portion 35b to the central axis P2. The angle θ2 between each of the imaginary lines M4 to M6 and the right adjacent one of the imaginary lines N4 to N6 is 60°. As shown in FIG. 7, when the second rotor 35 is viewed from the front side, the phase of the second straight portion 35b is displaced clockwise with respect to the first straight portion 35a about the central axis P2 by 60°. Hereinafter, “to displace the phase clockwise about the central axis P2 by 60° as viewed from the front side of the second rotor 35” is expressed as “clockwise phase displacement of 60°”.

As shown in FIG. 7, the lobes 31 and the recesses 32 of the coupling portion 30c of the first rotor 30 extend helically (counterclockwise) along the circumferential direction of the rotary shaft 21 from one to the other of the first straight portion 30a and the second straight portion 30b. The cross-sectional view of the coupling portion 30c in a direction perpendicular to the axial direction of the rotary shaft 21 has a three-lobe shape, as do the straight portions 30a, 30b, at any position in the axial direction of the rotary shaft 21.

Similarly, the lobes 36 and the recesses 37 of the coupling portion 35c of the second rotor 35 extend helically (clockwise) along the circumferential direction of the rotary shaft 23 from one to the other of the first straight portion 35a and the second straight portion 35b. The cross-sectional view of the coupling portion 35c in a direction perpendicular to the axial direction of the rotary shaft 23 has a three-lobe shape, as do the straight portions 35a, 35b, at any position in the axial direction of the rotary shaft 23.

FIGS. 8A and 8B schematically show a state where the phase of the first rotor 30 is slightly displaced from the original mounting position in the circumferential direction of the first rotary shaft 21. The illustrated example shows a state where the phase of the first rotor 30 is displaced counterclockwise. When such a phase displacement occurs, in the state shown in FIG. 8A, a clearance CLd at a closest portion S11 between the first rotor 30 and the second rotor 35 is smaller than the original clearance, and in the state shown in FIG. 8B, a clearance CLe at a closest portion S21 between the first rotor 30 and the second rotor 35 is greater than the original clearance. Such states also occur when another region S12 of the first rotor 30 and another region S22 of the second rotor 35 are closest to each other, and when another region S13 of the first rotor 30 and another region S23 of the second rotor 35 are closest to each other. In the state shown in FIG. 8A, the fluid leakage between the first rotor 30 and the second rotor 35 is small. In the state shown in FIG. 8B, the fluid leakage between the first rotor 30 and the second rotor 35 is increased. The state similar to that in FIG. 8A occurs three times while the rotors 30, 35 are rotated once, and the state similar to that in FIG. 8B occurs three times while the rotors 30, 35 are rotated once. Thus, suction pulsation having a fundamental order of three occurs.

The configuration in which the phase of the second straight portion 30b of the first rotor 30 is displaced counterclockwise with respect to the first straight portion 30a by 60°, and the phase of the second straight portion 35b of the second rotor 35 is displaced clockwise with respect to the first straight portion 35a by 60° reduces the suction pulsation having a fundamental order of three.

The present invention may also be embodied in the following forms.

The first rotor 30 and the second rotor 35 may be two-lobe rotors, which include two lobes 31, 36 and two recesses 32, 37 each provided between the lobes 31, 36. Also, the first rotor 30 and the second rotor 35 may be rotors having four or more lobes, which include four or more lobes 31 or 36, and four or more recesses 32 or 37 each provided between an adjacent pair of the lobes 31 or 36. The lobes 31, 36 and the recesses 32, 37 of the first straight portions 30a, 35a and the lobes 31, 36 and the recesses 32, 37 of the second straight portions 30b, 35b are preferably provided at equal intervals along the circumferential direction of the rotary shafts 21, 23.

The lengths of the rotary shafts 21, 23 at the first straight portions 30a, 35a and the second straight portions 30b, 35b along the axial direction may be different.

The coupling portions 30c, 35c do not need to extend helically along the circumferential direction of the rotary shafts 21, 23, but may extend straight from one to the other of the first straight portions 30a, 35a and the second straight portions 30b, 35b. The coupling portions 30c, 35c that extend straight suppress fluid leakage between the first rotor 30 and the second rotor 35 by engagement between the coupling portions 30c, 35c when the first rotor 30 and the second rotor 35 are rotated.

The present invention may be applied to a roots pump that uses rotors including three straight portions. In this case, X (the number of the straight portions) in the expression (1) may be three, and the angle θ of the phase displacement may be 20°. When three straight portions are assumed to be Z1, Z2, Z3 in the order from the front side of the rotor, in regard to the coupling portion between the straight portion Z1 and the straight portion Z2, one of the straight portion Z1 and the straight portion Z2 serves as the first straight portion, and the other one serves as the second straight portion. In regard to the coupling portion between the straight portion Z2 and the straight portion Z3, one of the straight portion Z2 and the straight portion Z3 serves as the first straight portion, and the other one serves as the second straight portion.

Number	Date	Country	Kind
2007-029183	Feb 2007	JP	national
2008-025310	Feb 2008	JP	national

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