Screw vacuum pump

Abstract: To provide a screw vacuum pump which is compact and yet capable of attaining a high degree of vacuum. A screw vacuum pump having a pair of male and female rotors 7 and 7A rotating in mesh with each other around two parallel axes, respectively, and a casing 1 for accommodating the two rotors 7 and 7A, the casing 1 having a suction port 8b and a discharge port 9b, the screw vacuum pump further having a process of sucking a gas from the suction port 8b into a space defined between the rotors 7 and 7A, a process of transferring the gas, a process of compressing the gas, and a process of discharging the gas from the discharge port 9b, wherein the suction port 8b is closed early, thereby providing an expansion process between the suction and transfer processes.

Description: The present invention relates to a screw vacuum pump and, more particularly, to a screw vacuum pump which is designed so that it is possible to raise the ultimate pressure. There has heretofore been one type of screw vacuum pump which has a pair of male and female rotors rotating in mesh with each other around two parallel axes, respectively, and a casing for accommodating the two rotors, the casing having a suction port and a discharge port. This type of pump includes: (A) screw vacuum pumps which have a process of sucking a gas from the suction port into a space defined between the rotors, and a process of compressing the gas inside the rotors; and (B) screw vacuum pumps which have a process of transferring the sucked gas between the suction and compression processes. All the above-described conventional screw vacuum pumps are arranged such that the suction port is closed when the space volume reaches a maximum. The type (A) of screw vacuum pump suffers from the problem that since the number of groove spaces present between the discharge and suction ports is small, the gas leaks to the suction side, and it is therefore impossible to attain a high degree of vacuum. In the type (B) of screw vacuum pump, the rotor wrap angle is increased (i.e., the rotor length is increased) to provide a transfer section inside the rotors, thereby increasing the number of groove spaces present between the discharge and suction ports. Therefore, this type of screw vacuum pump has the disadvantage that the axial length of the rotors increases, resulting in an increase in the overall size of the pump, although a high degree of vacuum can be attained. Document GB-A-2077951 discloses a screw pump which comprises a male rotor and female rotor rotating in mesh with each other around two parallel axes, respectively, and a casing for accommodating said two rotors. The casing has a suction port and a discharge port. Groove spaces are formed between the casing and the rotors. Gas is sucked from the suction port into the groove spaces and is compressed inside the groove spaces. The compressed gas is discharged from the discharge port. According to a first embodiment of this known screw pump the suction port has a fixed geometry and is closed when the volume of the respective groove space reaches a maximum. According to a second embodiment of this known screw pump, a slide valve is provided at the suction side by means of which the capacity of the screw pump can be regulated. It is an object of the present invention to provide a screw vacuum pump which is designed such that leakage of gas in the direction from the discharge side to the suction side is reduced so that a high degree of vacuum is obtained. This object is achieved by the screw vacuum pump according to claim 1. Further developments of the invention are defined in claims 2 and 3. In the above-described screw vacuum pump, it is essential in order to attain a high degree of vacuum to provide as many closed groove spaces as possible in between the discharge and suction ports and increase the number of seal lines to thereby reduce the leakage of gas to the suction port during the compression process. In the present invention, the number of closed groove spaces is increased by closing the suction port early, thereby providing an expansion process between the suction and transfer processes. Therefore, the transfer section can be shortened (in other words, the rotor length can be shortened). In addition, groove spaces where the pressure is lower than the suction pressure are provided in between the suction port and groove spaces undergoing the transfer and compression processes. Accordingly, it is possible to effectively prevent leakage of gas to the suction port. If screw vacuum pumps having the above-described arrangement and operation are connected in series in a multi-stage structure, a high degree of vacuum can be attained. In addition, if the pumping speed of each vacuum pump is set to be approximately equal to or higher than that of the preceding vacuum pump, there will be no rise in the gas in a passage connecting a pair of adjacent pumps at the time, for example, of evacuation of a gas of atmospheric pressure. Thus, it is possible to prevent the driving machine from being overloaded and hence possible to improve the reliability of the vacuum pump. Embodiments of the present invention will be described below with reference to the accompanying drawings. Fig. 1 shows the way in which a male rotor and a female rotor are in mesh with each other in a view developed in the circumferential direction of the rotors; and Fig. 2 is a sectional side view showing the structure of the screw vacuum pump according to the present invention. The screw vacuum pump has a casing comprised of a main casing 1 and a discharge casing 2, and a pair of male and female rotors 7 and 7A, which are rotatably supported by respective bearings 5a and 5b in a space defined between the main and discharge casings 1 and 2. The male and female rotors 7 and 7A are sealed off from lubricating oil used for the bearings 5a and 5b by respective shaft seals 6a and 6b. The male rotor 7 is, for example, rotated by an electric motor (not shown) through a speed change gear (not shown), while the female rotor 7A is rotated through a timing gear 10 with a small clearance between the same and the male rotor 7. A gas that is sucked in from a suction opening 8a is introduced through a suction port 8b into one of a plurality of groove spaces that are defined by the main casing 1 and the two rotors 7 and 7A. That is, the gas undergoes suction and compression processes and is then discharged from a discharge opening 9a through a discharge port 9b. More specifically, the gas undergoes a process for sucking the gas from the suction port 8b into the groove spaces defined by the rotors 7 and 7A, a process for expanding the gas sucked, a process for transferring the gas, and a process for compressing the gas inside the rotors 7 and 7A, and the gas is then discharged from the discharge opening 9a through the discharge port 9b. Fig. 1 shows the way in which the male and female rotors 7 and 7A are in mesh with each other in a view developed in the circumferential direction of the rotors. In Fig. 1, reference symbols A1 to A9 and B1 to B9 denote pairs of corresponding groove spaces of the rotors 7 and 7A. The groove spaces A1 and B1 are undergoing the process of sucking the gas from the suction port 8b; the groove spaces A2, A3, B2 and B3 are undergoing the process of expanding the gas sucked; the groove spaces A4, A5, A6, B4, B5 and B6 are undergoing the process of transferring the gas; the groove spaces A7, A8, B7 and B8 are undergoing the process of compressing the gas; and the groove spaces A9 and B9 are undergoing the process of discharging the gas from the discharge port 9b. As shown in Fig. 1, in the screw vacuum pump of this embodiment, the size of a wall portion 30 of the main casing 1 is increased so that the suction port 8b is closed early, thereby increasing the number of groove spaces between the suction and discharge ports, and thus providing the groove spaces A2, A3, B2 and B3, which are in the expansion process, and the groove spaces A4, A5, A6, B4, B5 and B6, which are in the transfer process, in between the groove spaces A1 and B1, which are in the suction process, and the groove spaces A7, A8, B7 and B8, which are in the compression process. More specifically, in the screw vacuum pump of this embodiment the suction port is closed early, thereby increasing the number of closed groove spaces, that is, providing the groove spaces A2, A3, B2 and B3, without increasing the rotor length between the discharge and suction ports 9b and 9a. Therefore, even if the number of groove spaces which are in the transfer section is reduced by shortening the rotor length, it is possible to ensure the same number of groove spaces as that in the prior art in between the discharge and suction ports 9b and 9a. Thus, the screw vacuum pump can be made compact without lowering the performance. In addition, groove spaces (that is, the groove spaces A2, A3, B2 and B3, which are in the expansion process, and the groove spaces A4, A5, A6, B4, B5 and B6, which are in the transfer process) where the pressure is lower than the suction pressure are provided in between the suction port 8b and groove spaces (that is, the groove spaces A7, A8, B7 and B8) undergoing the compression process. Accordingly, it is possible to effectively prevent leakage of gas to the suction port 8b. Although the above-described embodiment shows the arrangement and operation of a single screw vacuum pump, it should be noted that a plurality of screw pumps having the above-described arrangement may be arranged in series to form a multi-stage pump apparatus by connecting the suction opening of each pump to the discharge opening of the preceding one. With this arrangement, a high degree of vacuum can be attained. In the case of such a multi-stage pump apparatus, the pumping speed of each screw vacuum pump is set to be either approximately equal to or higher than that of the preceding pump. With this arrangement, there is no occurrence of such an undesirable phenomenon that the gas is compressed between a pair of adjacent vacuum pumps at the time, for example, of evacuation of a gas of atmospheric pressure. Thus, there is no possibility of each vacuum pump being overloaded.

Claims: A screw vacuum pump, which comprises a male rotor (7) and a female rotor (7A) rotating in mesh with each other around two parallel axes, respectively, and a casing (1, 2) for accommodating said two rotors (7, 7A), said casing (1, 2) having a suction port (8b) and a discharge port (9b) and groove spaces (A1 to A9, B1 to B9) being formed between said casing (1, 2) and said rotors (7, 7A), wherein gas is sucked from said suction port (8b) into said groove spaces (A1 to A9, B1 to B9), wherein said suction port (8b) is closed early such that the sucked-in gas is expanded after the respective groove space has been closed, wherein closed groove spaces (A4 to A6, B4 to B6) are provided in which the gas is transferred without being expanded or compressed, said transfer of gas following said expansion of gas, wherein said gas is compressed inside said groove spaces, said compression of gas following said transfer of gas and wherein said compressed gas is discharged from said discharge port (9b). A pump apparatus comprising a plurality of screw vacuum pumps as defined in claim 1, which are connected in series in a multi-stage structure. A pump apparatus according to claim 2, wherein the pumping speed of each screw vacuum pump is at least approximately equal to that of a preceding screw vacuum pump.

Inventors: SHIMIZU NOBURU, YANAGISAWA KIYOSHI, SHIMIZU, NOBURU, YANAGISAWA, KIYOSHI
Applicants: EBARA CORP, EBARA CORPORATION
Application year: 1992
Main classification: F04C18
IPC codes: F04C25, F04C23, F04C29, F04C18