Screw type vacuum pump

Claim 2

A screw vacuum pump comprising: a male rotor (31) having a plurality of spiral lands and grooves and a shaft portion and operative to rotate about said shaft portion; a female rotor (32) having a plurality of spiral lands and grooves and a shaft portion and operative to rotate about said shaft portion while being maintained in meshing engagement with said male rotor; and casings (33 and 34) defining a space for containing said two rotors and providing a suction port (55) and a discharge port (57) communicating with said space; said two rotors each having a wrap angle related to the position of said suction port and the position of said discharge port; wherein the improvement comprises: a plurality of working chambers defined by said two rotors and said casings including a plurality of sealed working chambers out of communication with both the suction port and the discharge port, said plurality of sealed working chambers comprising a plurality of working chambers (62, 63) having their volume reduced when the two rotors rotate while being maintained in meshing engagement with each other, and a plurality of working chambers (52, 60) having their volumes kept substantially constant when the two rotors rotate while being maintained in meshing engagement with each other, said working chambers having their volume reduced and said working chambers having their volume kept substantially constant being separated from each other by meshing portions (58, 59, 61) of said two rotors.

Claim 3

A screw vacuum pump as claimed in Claim 1, wherein a sealing portion at one end of said working chambers having their volumes kept substantially constant is constituted by one of the meshing portions of the two rotors and a sealing portions at an opposite end thereof is constituted by a wall of the casing facing opposite end faces of the two rotors.

Claim 4

A screw vacuum pump as claimed in Claim 2, wherein the lands and grooves of said male rotor are smaller than those of the female rotor by two(2) lands and two(2) grooves.

Claim 5

A screw vacuum pump as claimed in Claim 2, wherein the lands and grooves of said male rotor are four(4) in number, and those of said female rotor are six(6) in number.

Claim 6

A screw vacuum pump as claimed in Claim 3, wherein the lands and grooves of said male rotor are four(4) in number, and those of said female rotor are six(6) in number.

Claim 7

A screw vacuum pump as claimed in Claim 1, wherein said male rotor has a wrap angle ΨM of the lands expressed by the following formula: where a is the rotational angle of the rotor through which the rotor rotates from the time one of the working chambers is brought into communication with the discharge port until the time the volume of the working chamber becomes nil.

Claim 8

A screw vacuum pump as claimed in Claim 6, wherein the male rotor has a wrap angle ψ M of the lands which is 650°, and the female rotor has a wrap angle of the lands suitable to bring the female rotor into meshing engagement with the male rotor.

Claim 9

A screw vacuum pump as claimed in Claim 1, wherein the wrap angle ψ M of the lands of said male rotor is less than 650°, and the female rotor has a wrap angle of the lands suitable to bring the female rotor into meshing engagement with the male rotor.

Claim 10

A screw vacuum pump as claimed in Claim 4, wherein the wrap angle WM of the lands of said male rotor is less than 650°, and the female rotor has a wrap angle of the lands suitable to bring the female rotor into meshing engagement with the male rotor.

Claim 11

A screw vacuum pump as claimed in Claim 9, wherein said male rotor has a wrap angle ψ M of the lands which is about 600°.

Claim 12

A screw vacuum pump comprising: a male rotor having a plurality of spiral lands and grooves and a shaft portion and operative to rotate about said shaft portion; a female rotor having a plurality of spiral lands and grooves and a shaft portion and operative to rotate about said shaft portion while being maintained in meshing engagement with said male rotor, said lands of said female rotor being greater by one land than those of said male rotor; and casings defining a space for containing said two rotors and providing a suction port and a discharge port communicating with said space; wherein the improvement comprises: a plurality of working chambers defined by said two rotors and casings comprising a plurality of sealed working chambers out of communication with both the suction port and the discharge port, said sealed working chambers being at least two in number for one of said grooves of each said rotor and located along each said groove and one of said at least two sealed working chambers being a working chamber having its volume varied as said two rotors rotate while being in meshing engagement with each other and the rest of said at least two sealed working chambers being working chambers undergoing substantially no change in volume when said two rotors rotate, said sealed working chamber having its volume varied as said two rotors rotate and said sealed working chambers undergoing substantially no change in volume as said two rotors rotate being separated from each other by meshing portions of said two rotors.

Claim 13

A screw vacuum pump comprising: a male rotor having a plurality of spiral lands and grooves and a shaft portion and operative to rotate about said shaft portion; a female rotor having a plurality of spiral lands and grooves and a shaft portion and operative to rotate about said shaft portion while being maintained in meshing engagement with said male rotor, said lands of said female rotor being greater by one land than those of said male rotor; and casings defining a space for containing said two rotors and providing a suction port and a discharge port communicating with said space; wherein the improvement comprises: a plurality of working chambers defined by said two rotors and casings comprising a plurality of sealed working chambers out of communication with both the suction port and the discharge port, said sealed working chambers being at least two in number for one of said grooves of each said rotor and located along each said groove and one of said at least two sealed working chambers being a working chamber having its volume varied as said two rotors rotate while being in meshing engagement with each other and the rest of said at least two sealed working chambers being working chambers undergoing substantially no change in volume when said two rotors rotate, said sealed working chamber having its volume varied as said two rotors rotate and said sealed working chambers undergoing substantially no change in volume as said two rotors rotate being separated from each other by meshing portions of said two rotors.

Claim 14

A screw vacuum pump as claimed in Claim 11, wherein said plurality of working chambers defined by the grooves of the rotors are a pair of working chambers, one of which is a working chamber having its volume vary when the rotors rotate, and the other is a working chamber having its volume kept substantially constant when the rotors rotate.

Claim 15

A screw vacuum pump as claimed in Claim 11, wherein a sealing portion at one end of said working chambers having their volumes kept substantially constant is constituted by one of the meshing portions of the two rotors and a sealing portion at an opposite end thereof is constituted by a wall of the casing facing opposite end faces of the two rotors.

Claim 16

A screw vacuum pump as claimed in Claim 12, wherein a sealing portion at one end of said working chambers having their volumes kept substantially constant is constituted by one of the meshing portions of the two rotors and a sealing portion at an opposite end thereof is constituted by a wall of the casing facing opposite end faces of the two rotors.

Claim 17

A screw vacuum pump as claimed in Claim 12, wherein the lands and grooves of the male rotor are five(5) in number, and those of the female rotor are six(6) in number.

Claim 18

A screw vacuum pump as claimed in Claim 12, wherein said male rotor has a wrap angle of the lands expressed by the following formula: where a is the rotational angle of the rotor through which the rotor rotates from the time one of the working chambers is brought into communication with the discharge port until the time the volume of the working chamber becomes nil.

Claim 19

A screw vacuum pump as claimed in Claim 15, wherein the wrap angle ψ M of the lands of said male rotor is about 525°, and the wrap angle of the teeth of the female rotor is suitable to bring the female rotor into meshing engagement with the male rotor.

Claim 20

A screw vacuum pump as claimed in Claim 15, wherein the wrap angle ψ M of the lands of the male rotor is smaller than 525° by an angle within an angle of one lands of the female rotor, and the female rotor has a wrap angle of the lands suitable to bring the female rotor into meshing engagement with the male rotor, said female rotor having a portion of its end face on the suction side closed by one of said casings.

Claim 21

A screw vacuum pump as claimed in Claim 18, wherein the wrap angle V m of the lands of the male rotor is 450°, and the female rotor has a wrap angle of the lands suitable to bring the female rotor into meshing engagement with the male rotor.