OIL FREE SCREW COMPRESSOR
BACKGROUND OF THE INVENTION
1. Field of the Invent ion
The present invention relates to an oil free screw compressor in which
oil is prevented from entering a compression chamber formed by a pair of male
screw rotor and a female screw rotor.
2. Description of the Related Art
There are two types of screw compressors in which a pair of screw rotors
are combined to form a compression chamber. One is an oil-injected screw
compressor in which a lubricating oil is supplied to a compression chamber
in order to cool the compressor and to enable the smooth drive of the pair
of rotors. The other is an oil free screw compressor which avoids the contact
of a pair of rotors by driving them via timing gears and prevents oil from
entering a compression chamber.
In industrial fields such as electronics, food, and chemistry where
mixing of oil is regarded unfavorable, there is a great demand for oil free
screw compressors. However, it is said that an electric power expense of
an air compressor occupies as much as about 25% of electric power expenses
in a plant facility. Therefore, a compressor with little power consumption
and high energy efficiency is called for.
Japanese Patent Unexamined Publication No. 63-45488 discloses an
example of the conventional oil free screw compressor described above.
According to the screw compressor of Japanese Patent Unexamined Publication
No. 63-45488, in order to avoid a problem that the power of the compressor

increases when part cf the cooled discharge gas is returned to a suction side,
at a portion cioser to the discharge port side than a position of a wall part
of a rotor chamber in communication with the discharge port and, at the same
time, in a space of the rotor chamber near the discharge port or at the
discharge port immediately after the start of discharging, branch passages
branched on the exit side of discharge gas cooling means in the discharge
passage is allowed to merge.
Further, although it is not an oil free screw compressor, an example
of the oil-injected screw compressor is described in Japanese Patent
Unexamined Publication No. 2008-82273. According to the compressor of
Japanese Patent Unexamined Publication No. 2008-82273, in order to suppress
the increase in power consumption etc., a recess is formed in a wall face
opposed to an end face on the rotor discharge side of a casing. Further,
a compression chamber is brought into communication with the recess
immediately before the compression chamber is isolated from the discharge
port and such communication is kept until a volume of the compression chamber
becomes substantially zero.
In the oil free screw compressor according to Japanese Patent
Unexamined Publication No. 63-45488, a discharged gas is cooled at a position
as close as possible to the discharge port so that a discharge temperature
is kept low and a rise in suction pressure is suppressed to reduce power.
However, in the oil free screw compressor of Japanese Patent Unexamined
Publication No. 63-45488, enough consideration has not been given to an
increase in power caused by excessive compression etc. resulting from the
working gas remaining near the discharge port, namely, the excessive

compression etc. caused by a complex shape of the discharge port.
Moreover, the oil-injected screw compressor according to Japanese
Patent Unexamined Publication No. 2008-82273 permits a lubricating oil given
to bearings to enter the compression chamber, and oil is separatee; outside
the compressor. Therefore, a main body of the compressor is simply structured.
On the other hand, in order to prevent the oil from entering the compression
chamber, the oil free screw compressor requires a highly efficient shaft
sealing apparatus. As a result, since the rotor becomes longer, in order
to suppress vibration in a high-speed rotating apparatus, a length of the
casing in a depth direction of the end face is considerably restricted.
Further, when the invention as it is disclosed by Japanese Patent
Unexamined Publication No. 2008-82273 is applied to the oil free screw
compressor, according to a position or a shape of a slot formed ir. a depth
direction of the end face of the rotor, a volume of the compressed air on
the discharge side leaking into the compression chamber being ready for
suction may be increased. When the amount of leak increases, performance
of the oil free screw compressor is naturally degraded.
SUMMARY OF THE INVENTION
The present invention is made in view of the above problems of the
conventional art. Therefore, an object of the present invention is to prevent
the problems, in an oil free screw compressor, such as excessive compression
etc. which might be caused by closing of the discharge port before a volume
of the compression chamber becomes zero. Another object of the present
invention is to suppress an increase in power consumption caused by the

excessive compression etc. or an excessive rise in temperature of the
compressed air. Still another object cf the present invention is to realize
a discharge port capable of efficiently discharging a compressed air from
the discharge port until the timing with which the volume of the compression
chamber becomes zero with use of inexpensive manufacturing means. The
present invention aims to achieve at least one of the objects described above.
In order to achieve the above described object, according to a feature
of the present invention, there is provided an oil free screw compressor
comprising: a pair of male and female screw rotors in which two or more twisted
teeth are formed; a casing which receives both the rotors and in which a suction
port and a discharge port are formed; bearings which rotatably support shaft
end parts of both the rotors and are held in the casing; timing gears which
are mounted on one shaft end part of both the rotors and which allow both
the rotors to rotate in synchronism without contacting with each other; and
a shaft sealing apparatus which is disposed between each of the bearings and
teeth of the rotors and which prevents oil from entering a meshing portion
of the rotors, wherein there is provided a slot, next to the discharge port
of the casing, which comes into communication with the discharge port with
timing where a volume of a compression chamber formed by both the rotors and
the casing becomes substantially zero as both the rotors rotate.
In regard to the above feature, the slot preferably extends from a
periphery of a position where a tooth bottom circle of the female rotor
overlaps with a tooth top circle of the male rotor to a line which connects
centers of both the rotors. Further, it is desirable that the slot; extends
outward from a tooth cop part of the male rotor and its width is greater than

its depth in an axiai direction. Still further, the slot is preferably formed
by cutting to prevent the leak from a discharge side to a suction side.
According to the present invention, on the oil free screw compressor,
there is provided means to be in communication with the discharge port until
the volume of the compression chamber becomes substantially zero, which can
prevent problems such as excessive compression etc. and,can also suppress
an increase in power consumption to be caused by the excessive compression
etc. or an excessive rise in temperature of the compressed air. Further,
it is possible to efficiently discharge the compressed air from the discharge
port until the timing with which the volume of the compression chamber becomes
zero with use of an inexpensive manufacturing method.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a transverse cross sectional view showing a principal part
of one embodiment of an oil free screw compressor according to the present
invention;
Fig. 2 is a cross sectional view taken along line A-A of Fig. 1;
Fig. 3 is a cross sectional view taken along line B-B of Fig. I;
Fig. 4 is a cross sectional view taken along line C-C of Fig. 1; and
Figs. 5(a) and 5(b) are a top cross sectional view and a front cross
sectional view of one embodiment of the oil free screw compressor of the
present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
With reference to the drawings, one embodiment of the oil free screw •
compressor of the invention will be described. First, with reference to Figs.

5(a) and 5(b), an outline of the oil free screw compressor will explained.
Fig. 5(a) is the top cross sectional view of the oil free screw compressor
50 and Fig. 5(b) is the front cross sectional view. The oil free screw
compressor 50 has a pair of male screw rotor 1 and a female screw rotor 2,
which are received in bores formed in a casing 3. The bores are structured
such that two cylindrical spaces are provided next to each other in a direction
orthogonal to the axial direction, and their shaft centers are in parallel
with each other.
Shaft ends on one side of the rotors 1 and 2 are rotatably supported
by bearings 11 and 12, and shaft ends on the other side are rotatabiy supported
by bearings 13 and 14. The bearings 11 to 14 are held in the casing 3. A
lubricating oil is supplied to the bearings 11 to 14. The screw compressor
50 according to the present invention is an oil free compressor. Therefore,
in order to prevent the lubricating oil from entering a compression chamber
to be described later, shaft sealing apparatuses 31 to 34 are provided closer
to a central side than bearings in the axial direction.
A pinion gear 21 is mounted on one shaft end of the female rotor 2,
and the pinion gear 21 engages with a motor (not shown) . Timing gears 22
and 23 are mounted on the male and female rotors 1 and 2. The male and female
rotors rotate in synchronism through both the timing gears 22 and 23 engaging
with each other.
Middle portions of the male and female rotors 1 and 2 in the axial
direction have large diameters. The large-diameter portions of the rotors
1 and 2 are held v/ith little clearances between themselves ana inner wail
surfaces of the bores formed in the casing 3. On the large-diameter portions

of the rotors 1 and 2, there are formed two cr more slots or teetn in a spiral
manner. According to the present embodiment, the male rotor 1 has four teeth
and the female rotor 2 has six slots. While keeping a little clearance, the
teeth of the male rotor 1 engage with the slots of the female rotor 2.
On one side (right side in Fig. 5) of the male rotor i and the female
rotor 2, there is formed a suction port which leads the air taicen in through
the suction port to the compression chamber formed by both the rotors 1 and
2 and the casing 3. On the opposite side of both the rotors 1 and 2 in the
axial direction, there is formed a discharge port 4 which leads the compressed
air from the compression chamber formed by both the rotors 1 and 2 and the
casing 3 to the outside of the compressor.
With reference to a transverse cross sectional view of the screw
compressor 50 in Fig. 1 and Figs. 2 and 4 showing cross sections of parts
in Fig. 1, the discharge port 4 will be explained in detail. Fig. 2 is a
cross sectional view taken in the direction of arrows along line A-A of Fig.
1 and shows a periphery of the discharge port of the male rotor 1. Fig. 3
is a cross sectional view taken in the direction of arrows along line B-B
of Fig. 2 and shows a portion where the male and female rotors 1 and 2 engage
with each other and a periphery of the discharge port 4. Fig. 4 is a cross
sectional view taken in the direction of arrows along line C-C of the periphery
of the portion where the male and female rotors 1 and 2 engage with each other
and shows a horizontal cross section.
In the oil free screw compressor 50, air is introduced into the casing
3 through a suction pipe. Then, as the rotors 1 and 2 rotate, a volume of
the compression chamber formed by the pair of male and female rotors 1 and

2 and the casing 3 receiving both the rotors 1 and 2 is expamied and taken
inside the compressor. Subsequently, the compression chamber is completely
closed by the casing 3 around a point where the compression chamber comes
to have a maximum volume. As the rotors 1 and 2 further rotate, tne compression
chamber is contracted, thereby raising the air pressure.
Corresponding to a state where the compression chamber is.contracted
to a predetermined volume, the discharge port is formed in accordance with
tooth profiles of the rotors 1 and 2 in an end face portion in the axial
direction of the rotors 1 and 2 near the outer-diameter part. The compressed
air is discharged from this discharge port. In consideration of deformation
due to a level of processing accuracy, thermal expansion, and air pressure
as well as a bearing clearance, between the male rotor 1 and the female rotor
2 as well as between both the rotors 1 and 2 and the casing 3, clearances
are formed so that they do not come into contact with each other. In the
oil free screw compressor 50, when air escapes through the clearances, its
performance is degraded. Therefore, when the clearances are made as small
as possible, its performance is improved. Even so, the shape of the casing
3 cannot help becoming complex, and the casing is generally made by casting.
The suction port and the discharge port are also formed by casting. However,
a portion which has to be made with high precision is produced by machining.
In particular, as for the discharge port 4, in order to a void an increase
in power consumption due to excessive compression and a rise in temperature
of compressed air, it is desirable to form an opening so that the compressed
air can be discharged until the timing with which the volume of the compression
chamber becomes zero. However, it is difficult to form the opening by casting

or raachinir.g. Also, it nay increase the manufacturing cost. Therefore, the
clearances are allowed in the end faces of the rotors 1 and 2. The compressed
air escapes through the clearances. Therefore, even if the discharge port
is not kept opened until the timing with which the volume of the compression
chamber becomes zero, the excessive compression can be substantially
prevented.
The excessive compression can be prevented by the above conventional
method. However, since the compressed air constantly escapes from the end
face, the performance is inevitably degraded. In view of the above, according
to the present invention, the excessive compression is also prevented while
reducing the leak from the end face with use of a simple structure.
Since the explanation below is a theoretical one, the male rotor 1
and the female rotor 2 are regarded to be in contact through respective tooth
faces. As described above, in the actual oil free screw compressor, when
tooth faces come into contact with each other, friction, wear, vibration,
and noise are caused. Therefore, clearances are formed to such an extent
as the tooth faces not coming into contact with each other even when thermal
expansion is taken into account. However, the clearances are very small.
Therefore, in substantial consideration, it may be regarded that the tooth
faces come into contact with each other.
In Fig. 1, DIR indicates a direction in which the rotors 1 and 2 rotate.
When the male rotor 1 rotates in synchronism with the female rotor 2, an advance
face SF1, which is a tooth face of the male rotor 1 and a face which first
comes into contact with a tooth 8sur) face of the female rotor 2, engages with
a tooth face SF2 of the female rotor 2, which forms a compression chamber

7 between themselves and an inner wall face of the casing 3. As the rr.aie
and female rotors 1 and 2 rotate, the compression chamber 7 moves to a discharge
side from a suction side in the axial direction. At that time, the compression
chamber is divided by an imaginary contact line called a seal line between
the male rotor 1 and the female rotor 2. When a rear end of the seal line
reaches a suction end-wall part 3a of both the rotors 1 and 2, the volume
of the compression chamber 7 becomes substantially zero. Fig. 1 shows a state
where the volume of the compression chamber 7 has become zero.
That is, the male rotor 1 and the female rotor 2 are in contact with
each other at points PI, P2, and P3 on the tooth face. The compression chamber
is formed between the advance face SF1, whose both ends are partitioned by
the points Pi and P2, of the male rotor 1 and the tooth face SR2 of the female
rotor 2. On the other hand, another compression chamber 8 is formed by the
tooth face SRI opposed to an advance face PFl of the male rotor 1, the advance
face SF2 of the female rotor 2, and the points P2 and P3.
In the conventional art, the discharge port is formed corresponding
to the change in the shape of the compression chamber. As shown in Fig. 2,
a discharge passage 31 for connection to the compression chanioer is formed
near an end face on the discharge side of the rotors 1 and 2 in a lower portion
of the casing 3. As shown by a dashed line in Fig. 2, a side wall portion
of the casing 3 is opened to a profile formed by the anti-advance face SRI
of a tooth which engages after the male rotor 2, a tooth bottom circle of
the male rotor 2, the face SR2 opposite to the advance face of the teeth which
engage after the female rotor 2, a tooth bottom circle of the female rotor
2, and tooth top circles of the male ana female rotors 1 an:; 2.

Furthermore, according to the present invention, as described above,
in order to prevent the excessive compression without Degrading the
performance, a discharge port processing slot 6 is formed so as to be able
to discharge compressed air even when the volume of the compression chamber
becomes substantially zero. The processing slot 6 will be explained in detail
below.
As shown in Fig. 1, the processing slot 6 is located near a meshing
portion of a tooth top of the male rotor 1 and a tooth bottom of the female
rotor 2. In order to prevent the leak of air from the discharge side to the
suction side, the processing slot 6 is located on an outer side of the male
rotor 1 than the outer diameter of the tooth top so that the discharge port
4 does not come into communication with the compression chamber 8 in a state
of being ready for suction. Furthermore, in order that the compression
chamber 7 in a fully discharged state can be in communication with the
discharge port 4 as much as possible, the processing slot 6 is positioned
outside the outer diameter of the tooth top of the male rotor 1 by a clearance
of about d (see Fig. 2) between the end face of the rotor 1 on the discharge
side and the casing.3.
In order to reduce the leak of the compressed air to the shaft sealing
apparatus 34 of the female rotor 2, a width of the processing slot 6 is made
to be as small as possible. Although the depth of the processing slot 5,
namely, a dimension e in the axial direction is restricted because the shaft
sealing apparatus 34 is provided, it may be about half the width d of the
slot (see Fig. 4) . A length W of the processing slot 6 is set to be from
a corner portion of the above described discharge port 4, namely, a portion

where the tooth bottom part of the female rotor 2 overlaps with the tooth
top part of the male rotor 1 to a line which connects the shaft centers of
the male rotor 1 and the female rotor 2.
Specifications of the processing slot 5 is defined as described above
because of the following reasons: If the length W of the processing slot
5 is longer than that, namely, if it exceeds the line connecting the centers
of the male rotor 1 and the female rotor 2, it joins with a slot for starting
suction. Therefore, the compressed air is returned to the suction side. Also,
the depth e of the processing slot is defined by taking workability into
consideration. Further, if the width d of the processing slot 5 is widened
excessively, the slot becomes too close to a hole of the shaft sealing part
of the female rotor 2 and the compressed air flows into the shaft sealing
part. Because of these reasons, the dimension of the processing slot 5 is
defined as above.

WE CLAIM
1. An oil free screw compressor comprising:
a pair of male and female screw rotors in which two or
more twisted teeth are formed; a casing which receives both
the rotors and in which a suction port and a discharge port
are formed; bearings which rotatably support shaft end parts
of said both the rotors and are held in the casing; timing gears
which are mounted on one shaft end part of said both the two
rotors and which allow both the rotors to rotate in synchronism
without contacting with each other; and a shaft sealing
apparatus which is disposed between each of said bearings and
teeth of said rotors and which prevents oil from entering a
meshing portipn of the rotors,
wherein there is provided a slot, next to the discharge
port of said casing, which comes into communication with the
discharge port with timing where a volume of a compression
chamber formed by both the rotors and the casing becomes
substantially zero as said both the rotors rotate.
2. An oil free screw compressor according to claim 1,
wherein said slot extends from a periphery of a position where
a tooth bottom circle of said female rotor overlaps with a tooth
top circle of the male rotor to a line which connects centers
of both the rotors.
3. An oil free screw compressor according to claim 1,

wherein said slot extends outward from a tooth top part of saia
maie rotcr and its width is greater than its depth in an axial
direction.
4. An oil free screw compressor according to claim 2,
wherein said slot extends outward from a tooth top part of said
male rotor and its width is greater than its depth m an axial
direction.
5. An oil free screw compressor according to any one
of claims 1 to 4, wherein said slot is formed by cutting to
prevent the leak from a discharge side to a suction side.

To prevent, in an oil free screw compressor, problems
such as excessive compression etc. which might be caused by
a discharge port being closed before a volume of a compression
chamber becomes zero and to avoid an increase in power
consumption.
The oil free screw compressor comprises: a pair of male
and female screw rotors in which two or more twisted teeth are
formed; a casing which receives both the rotors and in which
a suction port and a discharge port are formed; and bearings
which rotatably support shaft end parts of both the rotors and
are held in the casing. There is provided a slot, next to the
discharge port of the casing, which comes into communication
with the discharge port with timing where a volume of a
compression chamber formed by both the rotors and the casing
becomes substantially zero as both the rotors rotate.