BACKGROUND OF THE INVENTION
Field of the invention
The present invention relates to a two-shaft
type screw fluid machine, and in particular to a two-
shaft type screw fluid machine improved in performance
by reducing an internal leak.
Description of related art
Conventionally, in order to regulate a
clearance between a male rotor and a female rotor
during operation, it has been known that the rotors are
formed in such a manner that a delivery end is smaller
than a suction end to make the clearance between the
rotors almost uniform during operation, because thermal
expansion is larger in the delivery end which becomes a
high temperature, as described in JP-A-57-159989, for
example.
In addition, in order to reduce noise and
vibration due to impact of lobes caused by separation
of flanks, it has been known that leads of the female
rotor and the male rotor are made different from each
other so that the male rotor and the female rotor
contact at their leading flanks (round sides) in the
suction end, and contact at their trailing flanks (flat
sides) in the delivery end, as described in JP-A-06-

159271, for example.
BRIEF SUMMARY OF THE INVENTION
In the prior art described above, in the case
of the rotor the delivery end of which is shaped in
accordance with thermal deformation, it is difficult to
ensure working accuracy in a region near the end. In
other words, because a cutting tool (hob or grindstone)
begins to bite near the rotor end, its deformation due
to working reaction force is different from that in
another portion, and therefore a machining error
becomes large, which requires a working method with
higher accuracy. Similarly, in the case of the rotors
having different leads, a lead error becomes large near
an end from which a tool starts to cut, and therefore
contact by meshing is likely to occur. For the purpose
of preventing this, a clearance between the rotors must
be set to be sufficiently large, which leads to
increase in leak to deteriorate the performance as a
fluid machine.
An object of the present invention is to
solve the above described problems in the prior art,
and maintain high performance by preventing mutual
contact of the rotors and keeping the clearance between
the rotors small. Additionally, another object of the
present invention is to provide high performance and
high reliability without using particularly high
accurate working method or working machine.

To achieve the above described objects, the
present invention provides a screw fluid machine in
which a pair of a female rotor and a male rotor meshes
with each other and rotates so that the volume of a
groove serving as a working chamber expands and
contracts in such a manner that gas is sucked from the
outside into the working chamber during expansion and
then the volume of the working chamber begins to
contract to compress the gas enclosed therein to a
predetermined pressure, wherein a clearance formed
between the female rotor and the male rotor is larger
in a predetermined region starting from a suction end
than that in another region.
According to the present invention, because a
flank is reduced in thickness only near a suction end,
a lead error increases in the thickness-reduced region
to prevent direct mutual contact of rotors while
keeping a clearance between the rotors small in a major
region. Therefore, it is possible to reduce a leak to
maintain high performance.
Other objects, features and advantages of the
invention will become apparent from the following
description of the embodiments of the invention taken
in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
Fig. 1 is a partial cross sectional view of a
female rotor and a male rotor according to one

embodiment of the present invention;
Fig. 2 is a side view of the female rotor
according to one embodiment;
Fig. 3 is a schematic view showing twisting
of the rotor according to one embodiment;
Fig. 4 is a schematic view showing twisting
of the rotor according to one embodiment;
Fig. 5 is a cross sectional view of a doby of
an oil-free screw compressor according to one
embodiment;
Fig. 6 is a graph of measurement of a lead
error of a rotor according to one embodiment; and
Fig. 7 is a perspective view showing seal
lines of a pair of a female rotor and a male rotor
according to one embodiment.
DETAILED DESCRIPTION OF THE INVENTION
One embodiment of the present invention will
be described with reference to Figs. 1 to 6.
Fig. 1 shows a part of a cross section of a
female rotor and a male rotor, and Fig. 2 is a side
view of a female rotor. Figs. 3 and 4 are schematic
views showing twisting of rotors. Fig. 5 shows a body
of an oil-free screw compressor. Fig. 6 shows a result
of measurement of a lead error of a rotor. Fig. 7
shows a pair of a female rotor and a male rotor with a
length of one pitch, and seal lines on flanks thereof.
A screw fluid machine has been widely used as

a compressor, an expander and a vacuum pump. By
rotating female and male rotors having thread-like
lobes meshing with each other, the volume of a groove
which serves as a working chamber expands and
contracts. In the case of compression, gas is sucked
from the outside into the working chamber during
expansion, and subsequently the volume of the working
chamber which is closed with almost the largest volume
inversely begins to contract to compress the gas
enclosed therein to a predetermined pressure.
Thereafter, a part of the working chamber is opened to
deliver the gas downstream.
The two female and male rotors have contours
referred to as a profile in a cross section
perpendicular to an axis. The profile is rotated, as
is moves in the axial direction, in proportion to its
moving distance to form a flank having a thread-like
outer surface which will be an envelope. The twist
directions of the female and male rotors are opposite
to each other so that the rotors mesh with each other.
Additionally, the shape of the profile in cross section
may not be completely uniform, and may be made to have
a small change in the axial direction for the purpose
of compensation of thermal deformation or the like.
Theoretically, the flanks of the female rotor
and the male rotor geometrically make line contact so
that seal lines 7 and 8 as shown in Fig. 7 are
coincident with each other, and that line defines a

part of the contour separating the working chambers.
However, for the purpose of smooth rotation, the rotors
are designed to be provided with a small clearance in
the actual region corresponding to the seal line so
that the rotors do not directly contact with each
other. The setting of the clearance is achieved in
such a manner that the lobe is made thin relative to
the theoretical profile, etc. Therefore, the actual
working chamber is not a completely closed space, and
there is a small clearance on the seal line.
The clearance makes a high pressure working
chamber and a low pressure working chamber communicate
with each other to cause an internal leak while the
high pressure working chamber and the low pressure
working chamber should be separated ideally, and
therefore the presence of the clearance which causes
the leak is not preferable for improving performance of
the fluid machine. Accordingly, the flanks of the
rotors are worked possibly with high accuracy so as to
minimize the clearance.
On the other hand, with respect to working
accuracy of the screw rotor, there is a tendency as
described below. When a lobe of a screw rotor is
machined by means of cutting, grinding or the like, a
tool shaves a surface of a raw material while moving
along a twisted groove of the rotor so that a desired
shape is obtained. Therefore, the working is generally
carried out from one end of the rotor toward the other

end, successively. During the working, because elastic
deformation due to working reaction force applied to
the tool and the raw material, and thermal deformation
due to working heat generation are accompanied, both
the material and the tool deform slightly in comparison
to those static state. This deformation may cause a
working error, but it is possible to reduce its
influence on flank accuracy by performing the working
while previously correcting the deformation.
However, near the end of the rotor from which
the working begins, the working reaction force is
unstable, the thermal deformation is different from
that in another portion where the working is
continuously performed, and therefore the correction of
the deformation is extremely difficult. Thus,
improvement of the working accuracy near the end of the
rotor is difficult in comparison to the other portion.
The structure of an air compressor will be
described with reference to Fig. 5, as a representative
of the oil-free screw fluid machine. A screw-type
vacuum pump and a screw-type expander have almost the
same structure basically, as well.
A male rotor 1 and a female rotor 2 having
twisted lobes are accommodated in a casing 3, and
rotate while meshing with each other. A groove of one
rotor is closed by an inner surface of the casing 3 and
the other rotor to form a plurality of working
chambers. One working chamber 4 among those is a space

shown by a part with hatching in Fig. 2.
The working chamber expands and contracts its
internal volume while moving in an axial direction by-
rotation of a pair of the rotors 1, 2. During volume
expansion, the working chamber communicates with a
suction port (not shown because it is located behind
the rotors in Fig. 5) to suck atmospheric air from the
outside. The communication with the suction port is
terminated when the volume becomes almost the largest,
and then the enclosed air is compressed to a
predetermined pressure with subsequent volume
contraction. The working chamber communicates with a
delivery port (not shown because it is located in front
of the rotors in Fig. 5) to deliver the compressed air.
One end of the shaft of the male rotor 1
projects outside the casing 3, and rotational power is
inputted from this end. Synchronous gears 5, 6 are
fixed on the shafts of the female rotor 2 and the male
rotor 1 and mesh with each other to transmit rotational
power from the male rotor 1 to the female rotor 2.
Because a backlash of the pair of the rotors 1, 2 are
set to be larger relative to that of the synchronous
gears 5, 6 so that the former surrounds the latter, the
flanks of the female rotor 2 and the male rotor 1 do
not directly contact with each other.
In the case of the oil-free screw compressor,
in contrast to an oil-cooled type screw compressor, oil
is not supplied to the rotor surfaces, and thus, if the

rotor surfaces which are generally made from metal
contact with each other with large relative velocity,
the rotor flanks are significantly damaged. In order
to prevent the damage and to maintain smooth rotation
without contact friction, a small clearance is provided
on a seal line between the rotors.
Although the female rotor 2 and the male
rotor 1 do not directly contact with each other, there
is a region referred to as a seal line on which the
rotors come close to each other with a small clearance
of about 10 to 100 (am therebetween when those are
meshed, as shown in Fig. 7. The seal line defines a
boundary which separates the working chambers in its
both sides. Therefore, the working chamber is not a
completely separated space, and there is a small
clearance between it and the adjacent working chamber
or the like. Because the clearance will become an
internal leak fluid path through which the compressed
air leaks into a working chamber with lower pressure,
performance of the compressor is degraded if the
clearance is too large. On the other hand, mutual
contact of the rotor flanks should be avoided as much
as possible and it is therefore required to keep the
clearance between the rotors at a proper value which is
the smallest in an order where there is not the
possibility of contact.
The clearance on the seal line has a larger
influence near the delivery end of the rotor. This is

because, near the suction end, the internal pressure
difference between the working chambers which oppose to
each other via the clearance is small and the internal
leak amount is relatively small. On the contrary, near
the delivery end, the pressure difference is large
because internal pressures of opposite working chambers
are suction pressure and delivery pressure,
respectively, and therefore the internal leak is larger
even if the size of the clearances are the same, which
leads to a larger influence on performance degradation.
In order to regulate the clearance on the
seal line, it is required to work the flanks with high
accuracy. Because the flanks are generally worked by
means of cutting or grinding, reaction force caused by
working acts on the tool and the rotor raw material
(work) to elastically deform those. In addition, heat
is generated by working, which results in thermal
deformation of the tool and the rotor raw material.
The elastic deformation and the thermal deformation are
factors of reducing the flank accuracy, however, in the
existing circumstances, the rigidity is relatively high
and the correction can be performed with almost
sufficient accuracy, except for near the end. In other
words, the groove of the rotor is generally worked by
means of cutting or grinding from one end toward the
other end, successively. Thus, the working reaction
force and the thermal deformation described above are
unstable in one end from which the working begins, and

the correction is very difficult there. Accordingly,
it is difficult to improve the accuracy near the one
end of the rotor in comparison to the other portion,
which is often embodied as a lead error, in particular.
In this embodiment, the tool is moved from
the suction side toward the delivery side in the
working process of the rotor. Therefore, lead accuracy
tends to deteriorate near the suction end. Thus, in a
region near the suction end in the entire length of the
rotor as shown in Fig. 2, the flank of either the
female rotor or the male rotor is reduced in thickness.
It is desirable that the thickness-reduced region is
about 1/10 to 1/4 of the entire length of the rotor.
Additionally, in view of the contact relationship
between the tool and the rotor raw material (work), the
thickness-reduced region may not be strictly defined in
the cross section perpendicular to the axis, but may
displace forward or backward in the axial direction
depending on the position on the flank as shown in Fig.
2.
The thickness-reduced part on the flank will
be described using the profile shown in Fig. 1. At
least one of a leading flank 11 of the male rotor, a
trailing flank 12 of the male rotor, a leading flank 13
of the female rotor and a trailing flank 14 of the
female rotor is reduced in thickness. The thickness-
reduced part and the thickness reduction amount are
selected in accordance with the accuracy and the lead

error tendency which vary depending on the rotor working method.
For example, the leading flank 13 and the trailing flank 14 of
the female rotor are reduced in thickness by about 50 to 100 um.
Although the thickness reduction method may be a method of
uniformly reducing the thickness in a direction orthogonal to th*
flank, or a method of increasing the cutting depth of the working
tool* thickness reduction in a rotational direction is
preferable in the case where a lead error is large in comparison
to a shape error of a profile. Thus* in at least one of the
leading flank and the trailing flank of at least one of the
female rotor and the male rotor, an index angle of the flank is
modified so that the flank is made thinner in a predetermined
region in comparison to another region. For example, when each
groove of the female rotor is worked* the leasing flank in the
region near the suction end shown in Fig. 2 is worked at a
rotational angle which is slightly advanced in the rotational
direction, while the trailing flank is worked at a rotational
angle which is slightly delayed.

When showing this along a helix, in the region near the
suction end, an angular phase of the leading flank 13a is
advanced in the rotational direction while an angular phase of
the trailing flank 14b is delayed, as shown in Fig. 3. Instead of
making the thickness reduction amount uniform in the region

near the suction end, it is desirable that the
thickness is largest at the suction end and gradually
decreases toward the delivery end, as shown in Fig. 4.
As described above, although unstable elastic
deformation and thermal deformation caused by the tool
starting to cut into the rotor raw material lead to
reduction in lead error, by locating it on a suction
side and reducing the thickness of the flank of the
female rotor in the rotational direction at the suction
end and in the region near the suction end, problems
such as damage of the rotor flanks due to contact and
interference of rotation due to friction do not occur
even if a flank of either the female rotor or the male
rotor or flanks of both rotors is/are slightly
displaced due to the lead error. In addition, because
the region where the rotor flank is reduced in
thickness is only the region near the suction end,
degradation of performance due to increase in clearance
between the rotors is minimized. Thus, a screw
compressor with improved performance and high
reliability can be achieved.
According to this embodiment, because the
profile of the male rotor is not changed and the
profile of the female rotor is changed only by slightly
shifting a phase in the rotational direction, it is
unnecessary to fabricate a new tool, the effectuation
is easy, and the working cost can be reduced.
In addition, an example in the case of

measuring a lead of the female rotor according to this
embodiment at normal temperatures will be described
with reference to Fig. 6.
When an axial position of the rotor is shown
on an abscissa axis, and a lead error is shown on an
ordinate axis in which a flank thicker direction is
upward and a flank thinner direction is downward, there
is a tendency such as a graph shown in Fig. 6. In
regard to the lead error, the difference between the
female rotor and the male rotor is more important than
the error with respect to a design value, and thus it
is not necessarily preferable to reduce an error.
Additionally, in the case of a rotor to which thermal
deformation compensation is applied by slightly
reducing the thickness of a profile on a delivery side
which becomes a higher temperature than a suction side,
a lead error shows a graph increasing from left to
right in which the flank is thicker on the suction
side. The thickness reduction near the suction end is
shown to be deviated downward in the graph because the
thickness reduction is in a thinner direction with
respect to the general lead error tendency over the
entire length of the rotor.
It should be further understood by those
skilled in the art that although the foregoing
description has been made on embodiments of the
invention, the invention is not limited thereto and
various changes and modifications may be made without

departing from the spirit of the invention and the
scope of the appended claims..

WE CLAIM:
1. A screw fluid machine in which a female rotor (2) and
a male rotor (1) each having twisted lobes including a groove
accommodated in a casing (3) and meshes with each other while
rotating, a plurality of working chambers (4) formed when one
groove of one of the rotors (1,2) being closed by an inner surface
of the casing (3) and the other rotor, the meshed rotating rotors
(1,2) enabling the volume of the working chambers (4) to expand
and contract in such a manner that gas is sucked from the atmos-
phere into the working chambers (4) during said expansion phase,
and then begins to contract so that the gas enclosed therein is
compressed to a predetermined pressure, characterized in that
a clearance is formed between the female rotor (2) and the
male rotor (1) which is larger in a predetermined region starting
from a suction end than that in the region proximal to a
discharge end.

2. The screw fluid machine as claimed in claim 1, wherein
a flank of at least one of the female rotor and the male rotor
has a reduced thickness in the predetermined region.
3. The screw fluid machine as claimed in claim 2, wherein
the thickness-reduced region of the flank has a length of 1/10 to
1/4 of the total length of one of the female rotor and the male
rotor.
4. The screw fluid machine as claimed in claim 1, wherein
at least one of a leading flank and a trailing flank of at least
one of the female rotor and the male rotor, has a reduced
thickness in the predetermined region as compared to that of the
region proximal to the discharge end.
5. The screw fluid machine as claimed in claim 4, wherein
an index angle of the flank is modified in the predetermined
region so that the flank thinner than that in the region proximal
to the distal end.

6. The screw fluid machine as claimed in claim 1, wherein
a leading flank in each groove of the female rotor is worked at
an advanced rotational angle in a rotational direction, and
wherein a trailing flank is worked at a delayed rotational angle
in the rotational direction, in the predetermined region.
7. The screw fluid machine as claimed in claim 1, wherein
the thickness of a flank of at least one of the female rotor and
the male rotor is largest at the suction end which gradually
decreases toward the discharge end, in the predetermined region.
8. The screw fluid machine as claimed in claim 1, wherein
a flank of at least one of the female rotor and the male rotor is
worked by moving a tool from the suction end toward the discharge
end.

In order to prevent mutual contact of rotors
while keeping a clearance between the rotors small to
maintain high performance, there is provided a screw
fluid machine in which a pair of a female rotor (2) and
a male rotor (1) meshes with each other and rotates so
that the volume of a groove serving as a working
chamber (4) expands and contracts in such a manner that
gas is sucked from the outside into the working chamber
(4) during expansion and then the volume of the working
chamber (4) begins to contract to compress the gas
enclosed therein to a predetermined pressure, wherein a
clearance formed between the female rotor (2) and the
male rotor (1) is larger in a predetermined region
starting from a suction end than that in another
region.