Screw vacuum pump with a reduced starting load

A screw vacuum pump is designed so that it is possible to reduce the load on the pump at the time of starting and evacuation of a gas under atmospheric pressure. The screw vacuum pump has male and female rotors rotating in mesh with each other around two parallel axes, respectively, and a casing for accommodating the rotors, the casing having a suction port and a discharge port, wherein the discharge port is formed so that V.sub.1 /V.sub.2 is in the range of 1.5 to 0.51, where V.sub.1 is a groove volume defined by the casing and the male and female rotors immediately after a gas has been trapped, and V.sub.2 is a groove volume immediately before the gas is discharged.

BACKGROUND OF THE INVENTION 
Field of the Invention 
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 reduce the load on the pump at the time of starting and 
evacuation of a gas under atmospheric 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. The 
operation of the screw vacuum pump comprises a process of sucking a gas 
from the suction port into a space defined between the rotors, a process 
of compressing the gas inside the rotors, and a process of discharging the 
gas from the discharge port. 
An advantageous way of obtaining a high degree of vacuum in the screw 
vacuum pump having the above-described arrangement is to increase the 
built-in volume ratio, that is, the compression ratio. However, in such a 
case, excessive power is needed at the time of starting and when a gas of 
atmospheric pressure is evacuated from the chamber during the top-speed 
operation. The following measures have heretofore been used in order to 
cope with the above-described problem: 
(1) A method wherein a throat is attached to the suction pipe to lower the 
pressure of the gas sucked into the pump. 
(2) A method wherein a gas relief mechanism is provided for groove spaces 
where high pressure is produced. 
(3) A method wherein the rotating speed is lowered by using an inverter or 
the like. 
(4) A method wherein a motor of large capacity is used. 
The conventional methods (1) to (4) suffer, however, from the following 
disadvantages: Method (1) causes a lowering in the pumping speed and hence 
takes much time to evacuate the chamber. Method (2) leads to an increase 
in cost and lacks reliability. Method (3) leads to an increase in cost 
because of the need for an inverter or the like to change the rotating 
speed. Method (4) lacks compactness and leads to an increase in cost 
because of the use of a motor of large capacity. 
SUMMARY OF THE INVENTION 
In view of the above-described circumstances, it is an object of the 
present invention to provide a screw vacuum pump which is designed so that 
it is possible to reduce the load on the pump at the time of starting and 
evacuation of a gas of atmospheric pressure and yet possible to obtain a 
high degree of vacuum. 
To solve the above-described problems, the present invention provides a 
screw vacuum pump having 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, wherein a rotor rotation angle at which the suction port 
closes a groove space formed by the casing and the male and female rotors 
is set at an angle at which the volume of the groove space has not yet 
reached a maximum, and the discharge port is formed so that V.sub.1 
/V.sub.2 is about 1, where V.sub.1 is a groove volume defined by the 
casing and the male and female rotors immediately after a gas has been 
trapped, and V.sub.2 is a groove volume immediately before the gas is 
discharged. 
In addition, the present invention is characterized in that a plurality of 
screw vacuum pumps having the above-described arrangement are connected in 
series in a multi-stage structure. 
In addition, the present invention is characterized in that the pumping 
speed of each screw vacuum pump is either approximately equal to or higher 
than that of the preceding screw vacuum pump. 
The power needed at the time of evacuation of a gas of atmospheric pressure 
can be reduced by setting the compression ratio at 1. However, in the 
prior art the trapping position of the suction port is set at a position 
where the groove volume reaches a maximum; therefore, if the compression 
ratio is reduced, the number of groove spaces present between the suction 
and discharge ports decreases, so that leakage of gas to the suction side 
increases, resulting in a lowering in the degree of vacuum attained. In 
contrast, if the suction port is closed early, the groove volume V.sub.1 
is relatively small, so that if the compression ratio is set at around 1 
(in the range of 1.5 to 0.51), the groove volume immediately before the 
groove space opens to the discharge port also decreases. It is therefore 
possible to delay the timing at which the groove space opens to the 
discharge port. Accordingly, although the compression ratio is around 1, a 
large number of groove spaces are present between the discharge and 
suction ports, and it is therefore possible to attain a high degree of 
vacuum.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiments of the present invention will be described below with reference 
to the accompanying drawings. FIGS. 2 and 3 show the structure of the 
screw vacuum pump according to the present invention. FIG. 2 is a 
sectional side view of the pump, and FIG. 3 is a sectional view taken 
along a plane perpendicular to the axes of a pair of male and female 
rotors. The screw vacuum pump has a main casing 1, 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. FIG. 2 also shows schematically a 
plurality of screw vacuum pumps P, Q, R connected in series in a 
multi-stage structure forming a pump apparatus. 
In the meantime, for example, the male rotor 7 is driven 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 
formed 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 a groove space that is defined by the main casing 1 
and the two rotors 7 and 7A, and the gas then undergoes expansion and 
compression processes as described hereafter before being discharged from 
a discharge opening 9a through a discharge port 9b. Reference numerals 3 
and 4 in FIG. 2 denote a gear cover and a cover, respectively. 
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 G1 and A2 to G2 denote 
pairs of corresponding groove spaces of the rotors 7 and 7A. A pair of 
groove spaces D1 and D2 define a maximum groove volume. In the prior art, 
since the trapping position of the suction port 8b is set at a position 
where the groove volume reaches a maximum, if the internal volume ratio 
(i.e., V.sub.1 /V.sub.2, where V.sub.1 is a groove volume defined by the 
casing and the male and female rotors immediately after a gas has been 
trapped, and V.sub.2 is a groove volume immediately before the gas is 
discharged) is reduced, the number of groove spaces present between the 
suction and discharge ports decreases. That is, the suction port 8b is 
closed at points 30a and 30b, and the discharge port 9b opens at points 
10a and 10b, so that there is only one pair of groove spaces D1 and D2 
between the discharge and suction ports 9b and 8b. Accordingly, leakage of 
gas to the suction side is large, so that it is difficult to attain a high 
degree of vacuum. 
In contrast, the present invention can attain a high degree of vacuum due 
to the following reason: If the suction port 8b is closed early, the 
groove volume V.sub.1 is relatively small; therefore, if the internal 
volume ratio is set at 1, the groove volume V.sub.2 immediately before the 
groove space opens to the discharge port 9b can also be made relatively 
small, so that it is possible to delay the timing at which the groove 
space opens to the discharge port 9b. Accordingly, although the internal 
volume ratio is 1, a large number of spaces are present between the 
discharge and suction ports 9b and 8b, and it is therefore possible to 
attain a high degree of vacuum. More specifically, the suction port 8b is 
closed at points 31a and 31b, while the discharge port 9b opens at points 
11a and 11b, and there are groove spaces C2-C1, D2-D1, E2-E1 and F2-F1 
therebetween. Thus, it is possible to prevent leakage of gas from the 
discharge side to the suction side. 
The operation of the present invention will next be explained with 
reference to FIG. 4. FIG. 4 shows the change of the groove volume V with 
respect to the angle .psi. of rotation of the male rotor 7. Pa denotes the 
atmospheric pressure, and the one-dot chain line shows the change of 
pressure in the groove space in the present invention, while the solid 
line shows the change of pressure in the groove space in the prior art. In 
FIG. 1, reference numerals 31a and 31b denote points at the male rotor 
rotation angle .psi.1; 11a, 11b denote points at the male rotor rotation 
angle .psi.3; 30a, 30b denote points at the male rotor rotation angle 
.psi.2; and 10a, 10b denote points at the male rotor rotation angle 
.psi.2. 
In the prior art, while the groove volume V.psi. is increasing, that is, 
while the rotation angle is in the range of .psi.0 to .psi.2, some groove 
spaces are open to the suction port 8b to allow a gas to be sucked. Near 
the rotation angle .psi.2 at which the groove volume V.psi. reaches a 
maximum value Vmax, the suction port 8b is closed for these groove spaces. 
In the case where the compression ratio is greater than 1, the groove 
volume V.psi. decreases thereafter until the rotation angle reaches .psi.3 
at which angle a predetermined space pressure P is reached, and the gas in 
the groove spaces is compressed. At the rotational angle .psi.3, the 
groove spaces are open to the discharge port 9b, and the gas in the groove 
spaces is discharged at the discharge pressure (atmospheric pressure) Pa. 
With regard to the change of the space pressure P in the prior art during 
the rotation from the angle .psi.0 to the angle .psi.3, the space pressure 
P becomes higher than the suction pressure P0 from an angular position 
immediately after the rotation angle .psi.2 at which the suction port 8b 
is closed for certain groove spaces, thus causing leakage of gas to the 
suction side. In the case where the compression ratio is 1, the pressure 
changes according to the sequence of P0.fwdarw.P01.fwdarw.P2, so that the 
leakage of gas to the suction side increases furthermore. 
In contrast, in the present invention the suction port 8b is closed for the 
pair of groove spaces C1 and C2 at the rotation angle .psi.1 before the 
groove volume V.psi. reaches the maximum value Vmax to cut off the groove 
spaces C1 and C2 from the suction side. In consequence, while the rotation 
angle is in the range of .psi.1 to .psi.2, as the groove volume V.psi. 
increases, the space pressure P1 lowers as shown by the one-dot chain line 
P1a. Thereafter, the compression process starts. In a case where the 
compression ratio is 1, the space pressure P1 is maintained at a pressure 
P1b lower than the suction pressure P0 until the rotation angle reaches 
.psi.3 at which the groove volume V.psi. becomes approximately equal to 
the groove volume V.psi.1 at the rotation angle .psi.1. Accordingly, 
leakage of gas to the suction port can be prevented. Thus, since there is 
no rise in the pressure in the groove space, even when a sublimable 
process gas is to be evacuated, there is a small possibility of the gas 
becoming solid. Therefore, the reliability of the vacuum pump is improved. 
In the foregoing description, leakage of gas between the groove spaces is 
ignored for simplification of explanation. In actual practice, however, 
there are small clearances between the meshing portions of the male and 
female rotors and between the rotors and the casing, and there is 
therefore leakage of gas into the groove spaces from the discharge side, 
and the actual compression ratio exceeds 1. Accordingly, the design 
compression ratio may be set at a value greater than 1 with the leakage of 
gas taken into consideration. If the driving machine has a sufficiently 
large capacity, the compression ratio may be increased to delay the timing 
at which the groove space opens to the discharge port, thereby increasing 
the number of groove spaces present between the suction and discharge 
ports. 
Although the above-described embodiment shows an arrangement comprising a 
single screw vacuum pump, it should be noted that a plurality of screw 
pumps having the above-described arrangement may be connected in series in 
a multi-stage structure by connecting the suction opening 8a of each pump 
to the discharge opening 9a of the preceding one. In this case, if the 
pumping speed of each screw vacuum pump is set to be either approximately 
equal to or higher than that of the preceding pump, 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, the load can be reduced, and it is 
possible to attain a higher degree of vacuum. 
As has been described above, the present invention provides the following 
advantageous effects: 
(1) Since there are a large number of groove spaces between the discharge 
and suction ports, a high degree of vacuum can be attained. 
(2) Since there is no rise in pressure in the groove spaces, even when a 
sublimable process gas is to be evacuated, there is a small possibility of 
the gas becoming solid in the groove spaces, and the reliability of the 
vacuum pump is improved. 
(3) By setting the compression ratio at about 1, it becomes possible to 
reduce the power needed at the time of evacuation of a gas of atmospheric 
pressure. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.