Screw vacuum pump

Abstract: To provide a screw vacuum pump capable of reducting the load thereon at the time of evacuation of a gas of atmospheric pressure with a simple structure. A screw vacuum pump having a pair of male and female rotors 7 rotating in mesh with each other around two parallel axes, respectively, in a casing 1 so that a gas that is sucked in from a suction opening 8a is introduced through a suction port 8b into a groove space defined between the male and female rotors 7 and the casing and then discharged from a discharge opening 9a through a discharge port 9b, wherein a throttle plate 2 is provided upstream and near the opening of the suction port 8a.

Description: The present invention relates to a screw vacuum pump according to the preamble of claim 1 and 2. The following are conventional methods of reducing the load at the time of evacuation of a gas of atmospheric pressure in a screw vacuum pump which has a pair of male and female rotors rotating in mesh with each other around two parallel axes, respectively, in a casing: (1) A method wherein the rotating speed of the screw vacuum pump is lowered at the time of evacuation of a gas of atmospheric pressure, thereby reducing the load on the pump. (2) A method wherein a valve is provided at the suction side of the screw vacuum pump and the valve is throttled to reduce the load on the pump at the time of evacuation of a gas of atmospheric pressure. (3) A method wherein the screw vacuum pump is arranged in a two-stage structure comprising a pre-stage pump and a post-stage pump and only the post-stage pump is operated at the time of evacuation of a gas of atmospheric pressure. The above-described methods (1) to (3) of reducing the load on the pump at the time of evacuation of a gas of atmospheric pressure suffer from the following disadvantages: The load reducing method (1) needs an inverter or the like to change the rotating speed of the pump. The load reducing method (2) necessitates providing a valve at the suction side and also needs a controller for controlling the throttling of the valve. The load reducing method (3) shortens the lifetime of the machine because the pre-stage pump repeats start and stop at the time of evacuation of a gas of atmospheric pressure. From document JP-A 59-176490 a generic screw vacuum pump is known having a pair of male and female rotors rotating in mesh with each other around two parallel axes. The rotors are situated in a casing so that a gas that is sucked in from a suction opening is introduced through a suction port into a groove space defined between said male and female rotors and said casing. Afterwards the gas in discharged from a discharge opening through a discharge port. Document FR-A-2601083 discloses a multicell pump provided with a restriction element in the suction port. This restriction element is inserted into the suction pipe to throttle the flow of sucked fluid. This document does fail to give any indication that it might be advantageous to provide a screw vacuum pump with features of the disclosed multicell pump. The object underlying the invention is to provide a screw vacuum pump capable of reducing the load on the pump at the time of evacuation of a gas of atmospheric pressure with a simple structure. This object is solved by the features of claim 1 as well as of claim 2. According to claim 1 a throttle plate is provided upstream and near the opening of the suction port. This throttle plate is formed by a projecting part of the casing. According to claim 2 a throttle plate is provided upstream and near the opening of said suction port, wherein said throttle plate is formed as a member separated from said casing and is attached to the casing when the pump is assembled. Claims 3 and 4 describe further advantageous developments of the pumps according to claims 1 and 2. In the following preferred embodiments of the invention are described. Fig. 1 is a sectional side view showing the structure of the screw vacuum pump according to the present invention; Fig. 2 is a view seen from the arrow A-A in Fig. 1 and showing the configuration of a throttle plate; Fig. 3 is a view seen from the arrow A-A in Fig. 1 and showing the configuration of a throttle plate; Fig. 4 is a sectional side view showing another example of the arrangement of a suction port and its vicinities in the screw vacuum pump according to the present invention; Fig. 5 shows an example of a throttle plate which extends from a suction connecting pipe; t;ig-. 6 is a view seen from the arrow A-A in Fig. 1 and showing another example of the configuration of the throttle plate; Fig. 7 is a view seen from the arrow A-A in Fig. 1 and showing another example of the configuration of the throttle plate; and Fig. 8 is a diagram showing the position of the throttle plate and the change of the pressure in the suction port. One preferred embodiment of the present invention will be described below with reference to the accompanying drawings. Fig. 1 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 1 and a pair of male and female rotors 7, which are rotatably supported by respective bearings 5a and 5b in a space defined inside the casing 1. The male and female rotors 7 are sealed off from lubricating oil used for the bearings 5a and 5b by respective shaft seals 6a and 6b. The shaft of one rotor, for example, the male rotor 7, is connected to a shaft of a motor 4. In addition, a timing gear 10 is provided on the male rotor 7 so that the male rotor 7 and the female rotor (not shown) are rotated through the timing gear 10 with a small clearance between the two rotors 7. Reference numeral 3 denotes a motor casing. A gas that is sucked from a suction opening 8a is introduced through a suction port 8b into a groove space that is defined by the casing 1 and the two rotors 7 and then discharged from a discharge opening 9a through a discharge port 9b. A throttle plate 2 is provided upstream and near the opening of the suction port 8b. Figs. 2 and 3 are views seen from the arrow A-A in Fig. 1, each showing the configuration of the throttle plate 2. As illustrated, the throttle plate 2 is provided in such a manner as to close the opening of the suction port 8b. The throttle plate 2 may be formed by projecting a part of the casing 1. Alternatively, the throttle plate 2 may be formed as a member separate from the casing 1 and attached to it when the pump is assembled. The throttle plate 2 can be formed in the same way even in the case of a pump structure having a suction opening 8a which extends in the axial direction, as shown in Fig. 4. Further, the restrictor 2 may extend from a suction connecting pipe 11, as shown in Fig. 5. In this case, the restrictor 2 is united with the suction connecting pipe 11 by a throttle plate support 12. In the screw vacuum pump having the above-described structure wherein the throttle plate 2 is provided upstream and near the opening of the suction port 8b, even if a gas of atmospheric pressure flows at the time of evacuation, the throttle plate 2 causes a pressure drop, and since the throttle plate 2 is provided near the opening of the suction port 8b, the gas is sucked into the groove space before the pressure recovers. As a result, the suction pressure and volume flow rate of the pump decrease, so that the load on the pump can be reduced, as shown in Fig. 8. Referring to Fig. 8, when a gas of atmospheric pressure flows in, a pressure drop occurs at the downstream side of a position a of the throttle plate 2, and the pressure gradually recovers as the distance from the throttle plate 2 increases downstream. The suction port 8b is disposed at positions b to c shown in the figure. In the case of a screw vacuum pump having a two-stage structure comprising a pre-stage pump and a post-stage pump, if the upstream (pre-stage) pump is arranged in the above described structure that has the throttle plate 2, the discharge pressure thereof is also low at the time of evacuation of a gas of atmospheric pressure by virtue of the throttling effect. Accordingly, the suction pressure of the downstream (post-stage) pump is low and the flow rate is also small. Therefore, the load on the downstream pump can also be reduced. The same advantageous effects are also obtained in pumps wherein the suction port 8b' is configured so as to trap a sucked gas before the groove space defined between the rotors and the casing reaches a maximum (see JP, A, 4-l50488), as shown in Figs. 6 and 7.

Claims: A screw vacuum pump having a pair of male and female rotors (7) rotating in mesh with each other around two parallel axes, respectively, in a casing (1) so that a gas that is sucked from a suction opening (8a) is introduced through a suction port (8b) into a groove space defined between said male and female rotors (7) and said casing (1) and then discharged from a discharge opening (9a) through a discharge port (9b), characterized by a throttle plate (2) provided upstream and near the opening of said suction port (8a), wherein said throttle plate (2) is formed by a projecting part of said casing (1). A screw vacuum pump having a pair of male and female rotors (7) rotating in mesh with each other around two parallel axes, respectively, in a casing (1) so that a gas that is sucked from a suction opening (8a) is introduced through a suction port (8b) into a groove space defined between said male and female rotors (7) and said casing (1) and then discharged from a discharge opening (9a) through a discharge port (9b), characterized by a throttle plate (2) provided upstream and near the opening of said suction port (8a), wherein said throttle plate (2) is formed as a member separate from said casing (1) and attached to it when said pump is assembled. A screw vacuum pump, as defined in claim 1 or 2, wherein said opening of the suction port (8a) extends in the axial direction, said throttle plate (2) extends from a suction connecting pipe (11) and is united with said suction connecting pipe (11) by a throttle plate support (12). A screw vacuum pump as defined in any one of claims 1 through 3, wherein said pump has a two-stage structure comprising a pre-stage pump and a post-stage pump, and said pre-stage pump has said throttle plate (2).

Inventors: YANAGISAWA KIYOSHI, YANAGISAWA, KIYOSHI
Applicants: EBARA CORP, EBARA CORPORATION
Application year: 1992
Main classification: F04C29
IPC codes: F04C18, F04C25, F01C1, F04C29, F04C28