Rotary screw vacuum pump with pressure controlled valve for lubrication/sealing fluid

A screw vacuum pump which includes a pair of screw rotors meshing each other with a small gap defined therebetween. A closing valve is provided in an oil circulation line leading from an oil stripper to a rotor chamber accommodating the screw rotors. A pressure switch for detecting a suction pressure is connected to the closing valve. When the suction pressure is lower than a predetermined value (i.e., at high level of vacuum), the closing valve is opened to supply the oil into the rotor chamber.

BACKGROUND OF THE INVENTION 
The present invention relates to a screw vacuum pump provided with an oil 
stripper. 
FIG. 3 shows a known oil screw compressor including a compressor body 21 
rotatably accomodating a pair of male and female screw rotors meshing with 
each other, a discharge line 3 leading from a discharge port 2 of the 
compressor body 21, and an oil stripper 5 having a filter 4 therein and 
connected to the discharge line 3. The screw compressor further includes 
an oil return line 22 leading from the oil stripper 5 through an oil pump 
13 to oil supply portions such as bearings, shaft sealing portions and 
rotor chamber in the compressor body 21. The oil thus returned from the 
oil stripper 5 to the compressor body 21 via the oil return line 22 is 
discharged again from the discharge port 2. 
The oil supplied to the oil supply portions in the compressor body 21 for 
the purpose of lubrication, gas cooling, sealing, etc. is discharged 
together with a compressed gas to the discharge line 3. Then, the 
compressed gas is separated from the oil by the filter 4 in the oil 
stripper 5, and is fed from an upper portion of the oil stripper 5. On the 
other hand, the oil separated from the compressed gas is dripped to a 
bottom portion of the oil stripper 5, and is reserved in the oil stripper 
5. Then, the oil is fed by an oil pump 13 to the oil supply portions in 
the compressor body 21, and is then introduced to the discharge port 2. 
Thus, the oil is circulated between the compressor body 21 and the oil 
stripper 5. 
Assuming that an internal compression ratio .pi. i of the compressor body 
21 is .pi. i=7, a discharge pressure is 7 kg/cm.sup.2 G in the case that a 
suction pressure is the atmospheric pressure. Accordingly, the discharge 
pressure is constant, and a gas flow rate at the filter 4 in the oil 
stripper 5 is therefore constant. Designing the size of the oil stripper 5 
is dependent upon the constant gas flow rate. 
Also known is an oil-free screw vacuum pump utilizing the above-mentioned 
compressor body 21 to be supplied with no oil into the rotor chamber in 
the compressor body 21. The oil-free screw vacuum pump is used in the 
fields (eg., semiconductor and food industries) wherein reverse flow of 
foreign matter to a vacuum suction side is not permitted. In this vacuum 
pump, a gap between the screw rotors and a gap between each screw rotor 
and a wall of the rotor chamber are not sealed by oil. Therefore, it is 
necessary to rotate the screw rotors at very high speeds, so as to reduce 
a quantity of gas leakage through the gaps. 
In the other fields wherein reverse flow of foreign matter to the vacuum 
suction side is somewhat permitted, an oil screw vacuum pump similar to 
the compressor shown in FIG. 3 may be utilized. In the oil screw vacuum 
pump, the gap between the screw rotors and the gap between each screw 
rotor and the wall of the rotor chamber are sealed by the oil supplied 
into the rotor chamber. Therefore, the rotational speed of the screw rotor 
needs not to be made so high as in the above-mentioned oil-free type. 
FIG. 4 shows such an oil screw vacuum pump having the same construction as 
the compressor shown in FIG. 3. Referring to FIG. 4, air is sucked from a 
vacuum tank 11 by a vacuum pump body 21a, and is compressed in the pump 
body 21a. Then, a compressed gas is discharged through the oil stripper 5 
to the atmosphere. 
Referring to FIG. 5, a suction pressure of the vacuum pump shown in FIG. 4 
starts changing from 760 Torr (atmospheric pressure) to a predetermined 
partial vacuum which depends on the application of the vacuum tank 11. 
Assuming that a volumetric flow of air to be sucked into a suction port of 
the screw vacuum pump is represented by V.sub.1 (m.sup.3 /hr); a 
volumetric flow of air to be discharged from the discharge port 2 is 
represented by V.sub.2 (m.sup.3 /hr); a suction pressure is represented by 
P.sub.1 (Torr); and a discharge pressure is represented by P.sub.2 (Torr), 
the volumetric flow V.sub.1 is defined according to the screw vacuum pump, 
and the following equation holds. 
EQU V.sub.2 =V.sub.1 .multidot.(P.sub.1 /P.sub.2) 
At starting of the vacuum pump, the suction pressure P.sub.1 is 760 Torr 
which starts dropping, and the discharge pressure P.sub.2 is also 760 Torr 
which is constant. Therefore, the volumetric flow V.sub.2 at a point shown 
in FIG. 5 is expressed as follows: 
EQU V.sub.2 =V.sub.1 .multidot.(760/760) 
When the suction pressures are reduced to 100 Torr at a point b and 10 Torr 
at a point c as shown in FIG. 5, the volumetric flows V.sub.2 at the 
discharge port 2 are as follows: 
EQU V.sub.2 =V.sub.1 .multidot.(100/760) 
EQU V.sub.2 =V.sub.1 .multidot.(10/760) 
In this manner, the volumetric flow at the discharge port in the case of a 
vacuum pump is changed widely according to the suction pressure. 
As previously mentioned, the size of the oil stripper 5 is dependent upon a 
gas flow rate on the discharge side. Accordingly, in the case that the 
suction pressure at the point c shown in FIG. 5 is used when designing the 
oil stripper 5, an air flow rate on the discharge side at the point a is 
76 times that at the point c, and 7.6 times that at the point b. As a 
result, the oil is widely scatterd to the atmosphere side at points a and 
b. In contrast, if the suction pressure at the point a is used when 
designing the oil stripper 5, the size of the oil stripper 5 becomes very 
large. 
In the case of the compressor, since a gas flow rate on the discharge side 
is substantially constant, proper selection of the oil stripper is easy. 
However, in a case of the vacuum pump, the gas flow rate on the discharge 
side changes up to 50 times or 100 times. Because of such a large change 
in gas flow rate, proper selection of the oil stripper size is difficult 
in both the case of small flow rate and large flow rate. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a screw 
vacuum pump which improves exhaust efficiency and ensure proper selection 
of the oil stripper. 
According to the present invention, there is provided a screw vacuum pump 
comprising a pump body having a pair of male and female screw rotors 
meshing each other with a small gap defined therebetween, one of said 
screw rotors being driven, a pair of rotor shafts for mounting said screw 
rotors, a pair of synchronizer gears mounted on said rotor shafts and 
meshing each other to synnchronously rotate both said screw rotors, a 
rotor chamber for accommodating said screw rotors, and a plurality of 
lubricating portions including bearing portions for lubricating said rotor 
shafts; an oil stripper for separating a compressed gas from an oil; a 
discharge gas line leading from a discharge port of said pump body to said 
oil stripper; an oil circulation line leading from said oil stripper 
through said lubricating portions and said rotor chamber in said pump body 
to said discharge port of said pump body, said oil circulation line being 
branched to a first line leading to said lubricating portions and a second 
line leading to said rotor chamber; a closing valve provided in said 
second line; and a pressure switch for detecting a suction pressure of 
said pump body and opening said closing valve when the suction pressure 
detected by said pressure switch becomes a predetermined value or less. 
With this construction, rotation of one of the screw rotors which do not 
directly constant each other is transmitted to the other screw rotor 
through the synchronizer gears mounted on the rotor shafts. Accordingly, 
when the suction pressure is high or normal (i.e.) near atmospheric, the 
oil need not be supplied to the rotor chamber for the rotation of the 
screw rotors. On the other hand, only when the suction pressure is low 
(i.e., a high vacuum) to cause a problem of gas leakage through the gap 
between both the screw rotors and the gap between each screw rotor and the 
wall of the rotor chamber, the oil is supplied to the rotor chamber by 
opening the closing valve provided in the oil circulation line leading to 
the rotor chamber, so that a change in gas flow rate on the discharge side 
may be reduced. 
That is, only when the suction pressure is lower than the predetermined 
value, the oil is supplied to the rotor chamber, thereby reducing the gas 
leakage through the gaps and improving the exhaust efficiency of the pump. 
Further, the size of the oil stripper can be properly selected according to 
a narrow employable range of gas flow rate, thereby eliminating release of 
soot to the atmosphere in case of an excessively small size of the oil 
stripper and a problem in case of an excessively large size thereof. 
Other objects and features of the invention will be more fully understood 
from the following detailed description and appended claims when taken 
with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
There will now be described a preferred embodiment of the present invention 
with reference to the drawings. 
Referring to FIGS. 1 and 2 wherein the same reference numerals as in FIGS. 
3 and 4 denote the same parts, the discharge port 2 of a pump body 1 is 
connected through the discharge line 3 to the oil stripper 5 having the 
filter 4 therein. There is provided in the pump body 1 a pair of female 
and male screw rotors 6 and 7 meshing with each other with a small gap 
defined therebetween. That is, both the rotors 6 and 7 are maintained not 
to directly contact with each other. A pair of synchronizer gears 8 and 9 
meshing with each other are mounted on respective rotor shafts of the 
screw rotors 6 and 7. The male screw rotor 7 is driven to rotate the 
synchronizer gears 8 and 9, thereby to effect the synchronous rotation of 
both the screw rotors 6 and 7. 
A suction line 10 is connected between the pump body 1 and the vacuum tank 
11, and a pressure switch 12 is provided in the suction line 10 for 
detecting a suction pressure. 
An oil circulation line 17 is connected between the bottom of the oil 
stripper 5 and the pump body 1. An oil pump 13 is provided in the oil 
circulation line 17. Downstream of the oil pump 13 the oil circulation 
line 17 is branched to first lines 14 communicated with lubricating 
portions including bearing portions, shaft sealing portions and 
synchronizer gear portions in the pump body 1 and a second line 16 
communicated with a rotor chamber in the pump body 1. A solenoid valve 15 
is provided in the second line 16. The oil supplied to the lubricating 
portions and the rotor chamber is discharged again from the discharge port 
2 to the oil stripper 5. 
The pressure switch 12 provided in the suction line 10 is connected to the 
solenoid valve 15 provided in the second line 16 branched from the oil 
circulation line 17 and leading to the rotor chamber. When the suction 
pressure detected by the pressure switch 12 becomes equal to or less than 
a predetermined value, the solenoid valve (i.e., high level of vacuum) 15 
is opened to allow the oil to be supplied into the rotor chamber, thereby 
sealing the gap between the rotors 6 and 7 and the gap between each rotor 
and the wall of the rotor chamber. 
That is, only when the suction pressure is low, the oil is supplied to the 
rotor chamber. The air sucked from the vacuum tank 11 by the pump body 1 
is compressed to be discharged with the oil from the discharge port 2. 
Then, the oil is separated from the compressed air in the oil stripper 5, 
and is reused to be supplied to the pump body 1. 
On the other hand, when the suction pressure exceeds the predetermined 
valve, the solenoid valve (i.e., rear atmospheric pressure) 15 is closed 
to cut the supply of oil to the rotor chamber. Accordingly, the air sucked 
from the vacuum tank 11 is compressed in the pump body 1 and is then 
discharged from the discharge port 2 without oil in the rotor chamber in 
the same manner as in the oil-free screw vacuum pump. Then, only the 
compressed air passes through the oil stripper 5. 
Accordingly, the oil stripper 5 may be so designed as to be able to 
separate the oil from the compressed air in a narrow pressure region 
corresponding to a low structure pressure, that is, in a narrow gas flow 
rate region. 
As to the oil supply portions in the rotor chamber, the oil may be supplied 
to the meshing portion between both the rotors, either of the rotors, or 
different portions of both the rotors. 
Although the above preferred embodiment includes the oil pump, it may be 
eliminated according to the present invention. 
While the invention has been described with reference to specific 
embodiments, the description is illustrative and is not to be construed as 
limiting the scope of the invention. Various modifications and changes may 
occur to those skilled in the art without departing from the spirit and 
scope of the invention as defined by the appended claims.