Note: Descriptions are shown in the official language in which they were submitted.
CA 02504474 2007-04-30
IMPROVED SCREW TtOTOR DEVICE
BACKGROUND OF THE lN'VENTION
l. FIELn OF THE TNVENTION
This invention relates generally to rotor devices and, more particularly to
screw rotors.
2. DESC.(tIPTION OF RELATED ARf
Screw rotors are generally irnown to be used in compressors, expanders, and
pumps. For each
of these applications, a pair of screw rotors have helical threads and grooves
that interrnesh with
each other in a housing. For an expander, a pressurized gaseous working fluid
enters rhe rotors,
expands into the volume as worlc is taken out from at least one of the rotors,
and is discharged at a
lower pressure. For a compressor, work is put into at least one of the rotors
to compress the gaseous
working fluid. Similarly, for a pump, work is put into at least one of the
rotors to pump the liquid.
The working fluid, either gas or liquid, enters through an inlet in the
housing, is positively displaced
within the housing as the rotors counter-rotate, and exits through an outlet
in the housing.
The rotor profiles define sealing surfaces between the rotors themselves
between the rotors
and the housing, thereby sealiztg a volume for the worlcirxg fluid in the
housing. The profiles are
traditionally designed to reduce leakage between the sealing surfaces, and
special attention is given
to the interface between the rotors where the threads and grooves of one rotor
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intermesh with the grooves and threads of the other rotor. The meshing
interface between rotors
must be designed such that the threads do not lock-up in the grooves, and this
has typically
resulted in profile designs similar to gears.
However, a gQar tooth is primarily designed for strength and to prevent lock-
up as teeth
mesh with each other and are not necessarily optimum for the circumferential
sealing of rotors
within a housing. As discussed above, threads must provide seals between the
rotors and the walls
of the housing and between the rotors themselves, and there is a transition
from sealing around the
circumference of the housing to sealing between the rotors. In this
transition, a gap is formed
between the meshing threads and the housing, causing leaks of the working
fluid through the gap
in the sealing surfaces and resulting in less efficiency in the rotor system.
Some arcuate profile designs, including earlier known involute tooth designs,
improve the
seal between rotors by minimizing the gap in this transition region. However,
many of these
profiles still retain the characteristic gear profile with tightly spaced
teeth around the
circumference, resulting in a number of gaps in the transition region that are
respectively produced
by each of the threads, and some designs minimize the number of threads and
grooves and may
only have a single acme thread for each of the rotors, but these threads
typically have a wide
profile around the circumferences of the rotors which also result in larger
gaps in the transition
region. Single thread profiles can also result in imbalances in the rotors
when rotated at high
speeds and multiple thread profiles allow for leaks between the positive
displacement flow regions
bounded by the multiple threads. The leaks between multiple threads in these
rotors can also be
significant in prior art designs wlien the rotor length extends beyond a
single pitch of the threads.
Many of the prior art thread designs use multiple pitch threads and result in
additional leakage.
Additionally, many of these designs are based on multiple curves in a
lengthwise cross-section.
Multiple curves impose manufacturing constraints that adversely impact the
ability to manufacture
the rotors and to maintain close tolerances between the rotors.
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BRIEF SUMMARY OF THE INVENTION
It is in view of the above problems that the present invention was developed.
The
invention features a screw rotor device with helical threads on a male rotor
that mesh with the
identical number of corresponding helical grooves on a female rotor. The
intermeshing rotors are
rotatably mounted in a housing and have a pair of axes between the ends of the
housing. In one
aspect of the invention, the cross-sectional shape of the rotors can be
identical, i.e., twin rotors. In
another aspect of the invention, the cross-sectional shape of the rotors can
be quite different. In
either case, the screw rotor device has an identical number of threads (N) and
the twisting of the
cross-sectional shape along the respective rotor axes results in a helical
shape for each rotor that
intermeshes with the helical shape of the other rotor. Accordingly, the rotors
can generally be
referred to as having intermeshing helical element pairs. In other aspects of
the invention, the
helical element pairs can be, in the alternative or in any combination, a
phase-offset thread and a
corresponding phase-offset groove, a pair of single-pitch buttress threads, or
a pair of single-pitch
concave/convex threads.
The phase-offset helical threads on the male rotor mesh with the identical
number of
corresponding phase-offset helical grooves on the female rotor. In one aspect
of the phase-offset
helical threads, the helical groove can have a cut-back concave profile that
meshes with a
corresponding cut-in convex profile of the helical thread. The cut-back
concave profile
corresponds with a helical groove having a radially narrowing axial width at
the periphery of the
female rotor. In another aspect of the phase-offset helical threads and
corresponding grooves,
these helical element pairs can have the buttress thread profile, i.e., a
diagonal line between the
intermeshing rotors. The concave portion of the concave-convex thread is
formed by a path of the
maj or diameter arc on the thread other of the intermeshing rotors, whereas
the convex curve for the
intermeshing rotors can be defined by a slope of the diagonal lines along with
the diameter and arc
angle of the rotors' major diameter and minor diameter arcs. Additionally, the
maximum length of
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the rotors can be limited to a single pitch of the helical element pairs. The
features of the invention
result in an advantage of improved efficiency and manufacturability of the
screw rotor device.
Further features and advantages of the present invention, as well as the
structure and
operation of various embodiments of the present invention, are described in
detail below with
reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and form a part of the
specification, illustrate the embodiments of the present invention and
together with the description,
serve to explain the principles of the invention. In the drawings:
Figure 1 illustrates an axial cross-sectional view of a screw rotor device
according to the
present invention;
Figure 2A illustrates a detailed cross-sectional view of one embodiment of the
screw rotor
device taken along the line 2-2 of Figure 1;
Figure 2B illustrates a detailed cross-sectional view of another embodiment of
the screw
rotor device taken along the line 2-2 of Figure 1;
Figure 3 illustrates a detailed cross-sectional view of the screw rotor device
taken along
line 3-3 of Figure 1;
Figure 4 illustrates a cross-sectional view of the screw rotor device taken
along line 4-4 of
Figure l; and
Figure 5 illustrates a schematic diagrain of an alternative embodiment of the
invention.
Figure 6A illustrates a detailed cross-sectional view of the screw rotor
device taken along
line 6-6 of Figure 2A.
Figure 6B illustrates a detailed cross-sectional view of the screw rotor
device taken along
line 6-6 of Figure 2B.
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Figure 7A illustrates an axial cross-sectional view of another alternative
embodiment of
the screw rotor device according to the present invention
Figure 7B illustrates a lengthwise cross-sectional view of the screw rotor
device taken
along line 7B-7B of Figure 7A.
Figure 8 illustrates a lengthwise cross-sectional view of a screw rotor device
according to
yet another aspect of the present invention;
Figure 9 illustrates a cross-sectional view of the screw rotor device taken
along line 9-9 of
Figure 2;
Figure 10 illustrates an isometric view of a pair of twin rotors for the screw
rotor device;
Figure 11 illustrates a lengthwise cross-sectional view of the screw rotor
device taken
along line 11-11 of Figure 9;
Figure 12 illustrates the satne lengthwise cross-sectional view of the screw
rotor device
illustrated in Figure 11 after the twin rotors have been rotated approximately
90 ;
Figure 13A illustrates a cross-sectional view of an alternative twin rotor
embodiment for
the screw rotor device;
Figure 13B illustrates a lengthwise cross-sectional view of the alternative
twin rotor
embodiment taken along line 13B-13B of Figure 13A;
Figure 14A illustrates a cross-sectional view of anotller alternative twin
rotor embodiment
for the screw rotor device;
Figure 14B illustrates a lengthwise cross-sectional view of the alternative
twin rotor
embodiment taken along line 14B-14B of Figure 14A; and
Figure 14C illustrates the same cross-sectional view of the screw rotor device
illustrated in
Figure 14A after the twin rotors have been rotated approximately 60 .
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DETAILED DESCRIPTION OF THE INVENTION
Referring to the accompanying drawings in which like reference numbers
indicate like
elements, Figure 1 illustrates an axial cross-sectional schematic view of a
screw rotor device 10.
The screw rotor device 10 generally includes a housing 12, a male rotor 14,
and a female rotor 16.
The housing 12 has an inlet port 18 and an outlet port 20. The inlet port 18
is preferably located at
the gearing end 22 of the housing 12, and the outlet port 20 is located at the
opposite end 24 of the
housing 12. The male rotor 14 and female rotor 16 respectively rotate about a
pair of substantially
parallel axes 26, 28 within a pair of cylindrical bores 30, 32 extending
between ends 22, 24.
In the preferred embodiment, the male rotor 14 has at least one pair of
helical threads 34,
36, and the female rotor 16 has a corresponding pair of helical grooves 38,
40. The female rotor
16 counter-rotates with respect to the male rotor 14 and each of the helical
grooves 38, 40
respectively intermeshes in phase with each of the helical threads 34, 36. In
this manner, the
working fluid flows through the inlet port 18 and into the screw rotor device
10 in the spaces 39,
41 bounded by each of the helical threads 34, 36, the female rotor 16, and the
cylindrical bore 30
around the male rotor 14. It will be appreciated that the helical grooves 38,
40 also define spaces
bounding the working fluid. The spaces 39, 41 are closed off from the inlet
port 18 as the helical
tllreads 34, 36 and helical grooves 38, 40 intermesh at the inlet port 18. As
the female rotor 16 and
the male rotor 14 continue to counter-rotate, the working fluid is positively
displaced toward the
outlet port 20.
The pair of helical threads 34, 36 have a phase-offset aspect that is
particularly described
in reference to Figures 2A, 2B and 3 which show the cross-sectional profile of
the screw rotor
device through line 2-2, the two-dimensional profile being represented in the
plane perpendicular
to the axes of rotation 26, 28. The phase-offset aspect is also discussed
below in reference to
Figure 7A. The cross-section of the pair of helical threads 34, 36 includes a
pair of corresponding
teeth 42, 44 bounding a toothless sector 46. The phase-offset of the helical
threads 34, 36 is
defmed by the arc angle (3 subtending the toothless sector 46 which depends on
the arc angle a of
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either one of the teeth 42, 44. In particular, for phase-offset helical
threads, the toothless sector 46
must have an arc angle (3 that is at least twice the arc angle a subtending
either one of the teeth 42,
44. The phase-offset relationship between arc angle and arc angle a is
particularly defined by
equation (1) below:
Arc Angle (3 ? 2 Arc Angle a (1)
As illustrated in Figures 2A and 2B, the angle between ray segment oa and ray
segment
ob, subtending tooth 42, is arc angle a. According to the phase-offset
defmition provided above,
arc angle (3 of the toothless sector 46 must extend from ray segment ob to at
least to ray segment
oa', which would correspond to twice the arc of arc angle a, the minimum phase-
offset multiplier
being two (2) in equation 1. In the preferred embodiment, the arc an gle (3 of
the toothless sector 46
extends approximately five times arc angle a to ray segment oa", corresponding
to a phase-offset
multiplier of five (5). Accordingly, another two additional teeth could be
potentially fit on
opposite sides of the male rotor 14 between the teeth 42, 44 while still
satisfying the phase-offset
relationship with the minimum phase-offset multiplier of two (2).
For balancing the male rotor 14, it is preferable to have equal radial spacing
of the teeth.
An even number of teeth is not necessary because an odd number of teeth could
also be equally
spaced around male rotor 14. Additionally, the number of teeth that can fit
around male rotor 14 is
not particularly limited by the preferred embodiment. Generally, arc angle P
is proportionally
greater than arc angle a according to the phase-offset multiplier.
Accordingly, arc angle (3 of the
toothless sector 46 can decrease proportionally to any decrease in the arc
angle a of the teeth 42,
44, thereby allowing more teeth to be added to male rotor 14 while maintaining
the phase-offset
relationship. Whatever the number of teeth on the male rotor 14, the female
rotor has a
corresponding number of helical grooves. Accordingly, the helical grooves 38,
40 have a phase-
offset aspect corresponding to that of the helical threads 34, 36. Therefore,
the female rotor has
the same number of helical grooves 38, 40 as the number of helical threads 34,
36 on the male
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rotor, and the helix angle of the helical grooves 38, 40 is opposite-handed
from the helix angle of
the helical threads 34, 36.
In the preferred embodiment, Eacheach of the helical grooves 38, 40 preferably
has a cut-
back concave profile 48 and corresponding radially narrowing axial, widths
from locations
between the minor diameter 50 and the major diameter 52 towards the major
diameter 52 at the
periphery of the female rotor 16. The cut-back concave profile 48 includes
line segment jk
radially extending between the minor diameter 50 and the major diameter 52 on
a ray from axis 28,
line segment Im radially extending between the minor diameter 50 and the major
diameter 52, and
a minor diameter arc Ij circumferentially extending between the line segments
jk, Im. Line
segment jk is substantially perpendicular to major diameter 52 at the
periphery of the female rotor
16, and line segment Imn preferably has a radius Im combined with a straight
segment mn. In
particular, radius Im is between straight segment mn and minor diameter arc Ij
and straight
segment mn intersects major diameter 52 at an acute exterior angle cp,
resulting in a cut-back angle
0 defined by equation (2) below.
Cut-Back Angle (D = Right Angle (90 ) - Exterior Angle cp, (2)
The cut-back angle (D and the substantially perpendicular angle at opposite
sides of the
cut-back concave profile 48 result in the radial narrowing axial width at the
periphery of the
female rotor 16. In the preferred embodiment, the helical grooves 38, 40 are
opposite from each
other about axis 28 such that line segment jk for each of the pair of helical
grooves 38, 40 is
directly in-line with each other through axis 28. Accordingly, in the
preferred embodiment, line
segment Icjxj'k' is straight.
In the preferred embodiment of the present invention, the screw rotor device
10 operates as
a screw compressor on a gaseous working fluid. Each of the helical threads 34,
36 may also
include a distal labyrinth seal 54, and a sealant strip 56 may also be wedged
within the distal
labyrinth seal 54. The distal labyrinth seal 54 may also be formed by a number
of striations at the
tip of the helical threads (not shown). When operating as a screw compressor,
the screw rotor
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device 10 preferably includes a valve 58 operatively communicating with the
outlet port 20. In the
preferred embodiment, the valve 58 is a pressure timing plate 60 attached to
and rotating with the
male rotor 14 and is located between the male rotor 14 and the outlet port 20.
As particularly
illustrated in Figure 4, the pressure timing plate 60 has a pair of cutouts
62, 64 that sequentially
open to the outlet port 20. Between the cutouts 62, 64, the pressure timing
plate 60 forms
additional boundaries 66, 68 to the spaces 39, 41 respectively. As the male
rotor 14 counter-
rotates with the female rotor 16, boundaries 66, 68 cause the volume in the
spaces 39, 41 to
decrease and the pressure of the working fluid increases. Then, as the cutouts
62, 64 respectively
pass over the outlet port 20, the pressurized working fluid is forced out of
the spaces 39, 41 and the
spaces 39, 41 continue to decrease in volume until the bottom of the
respective helical threads 34,
36 pass over the outlet port.
Figure 5 illustrates an alternative embodiment of the screw rotor device 10
that only has
one helical thread 34 intermeshing with the corresponding helical groove 38
and preferably has a
valve 58 at the outlet port 20. As illustrated in Figure 5, the valve 58 can
be a reed valve 70
attached to the housing 12. In this embodiment, weights may be added to the
male rotor 14 and
the female rotor 16 for balancing. The helical groove 38 can have the cut-back
concave profile 48
described above, and the male rotor 14 again counter-rotates with respect to
the female rotor 16.
The alternative embodiment also illustrates another aspect of the screw rotor
device 10
invention. In this embodiment, the length of the screw rotor device 10 is
limited to a single pitch
of the helical thread 34 and groove 38. The pitch of a screw is generally
defined as the distance
from any point on a screw thread to a corresponding point on the next thread,
measured parallel to
the axis and on the same side of the axis. The particular screw rotor device
10 illustrated in Figure
5 has a single thread 34 and corresponding groove 38. Therefore, a single
pitch of the 34 and
groove 38 requires a complete 360 helical twist of the thread 34 and
corresponding groove 38. .
The present invention is directed toward screw rotor devices 10 having the
identical number of
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threads and grooves (N), and the helical twist required to provide the single
pitch is merely defined
by the number of threads and grooves (N = 1, 2, 3, 4, ...) according to
equation (3) below.
Single Pitch Helical Twist = 360 /N (3)
Of course, it will be appreciated that although the length of the screw rotor
device 10 is
limited to a single pitch, the pitch length can be changed by altering the
helix angle of the threads
and grooves. The pitch length increases as the helix angle steepens. The screw
rotor device 10
illustrated in Figure 1 has a pair of threads 34, 36 and a corresponding pair
of helical grooves 38,
40 (N=2). Therefore, a single pitch of these rotors would only require a 180
helical twist
(360 /2). However, it is evident that the screw rotor device 10, as
illustrated in Figure 1, has a
length slightly greater than two pitches. Therefore, for the given length of
the rotors, the helix
angle for the threads and grooves would have to increase for the rotors to
have a single pitch
length. For example, Figures 7A and 7B illustrate a screw rotor device 10 that
has a pair of
threads 34, 36 and a corresponding pair of helical grooves 38, 40 that are
limited to a 180 helical
twist. Accordingly, Figures 7A and 7B particularly illustrate rotor lengths
that are limited to the
single pitch of the threads 34, 36 and grooves 38, 40.
The screw rotor device 10 illustrated in Figure 7A also incorporates the phase-
offset
relationship into its design. The angle between ray segment oa and ray segment
ob, subtending
tooth 42, is arc angle a. According to the phase-offset definition provided
above, arc angle (3 of
the toothless sector 46 must extend from ray segment ob to at least to ray
segment oa', which
would correspond to twice the arc of arc angle a, the minimum phase-offset
multiplier being two
(2) in equation 1.
As particularly illustrated in Figure 3, the helical thread 34 preferably has
an cut-in convex
profile 72 that meshes with the cut-back concave profile 48 of the helical
groove 38. The cut-in
convex profile 72 has a tooth segment 74 radially extending from minor
diameter arc ab. The
tooth segment 74 is subtended by arc angle a and is further defined by
equation (4) below
according to arc angle 0 for minor diameter arc ab.
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Arc Angle a > Arc Angle 0 (4)
The phase-offset relationship defined for a pair of threads is also applicable
to the male
rotor 14 with the single thread 34, such that the toothless sector 46 must
have an arc angle (3 that is
at least twice the arc angle a of the single helical thread 34. The male rotor
14 circumference is
360 . Therefore, arc angle (3 for the toothless sector 46 must at least 240
and arc angle a can be
no greater than 120 . Similarly, for the pair of threads 34, 36, 60 is the
maximum arc angle a that
could satisfy the minimum phase-offset multiplier of two (2) and 30 is the
maximum arc angle a
that could satisfy the phase-offset multiplier of five (5) f or the preferred
embodiment. For
practical purposes, it is likely that only large diameter rotors would have a
phase-offset multiplier
of 50 (3 maximum arc angle a) and manufacturing issues may limit higher
multipliers.
The male rotor 14 and female rotor 16 each has a respective central shaft 76,
78. The
shafts 76, 78 are rotatably mounted within the housing 12 through bearings 80
and seals 82. The
male rotor 14 and female rotor 16 are linked to each other through a pair of
counter-rotating gears
84, 86 that are respectively attached to the shafts 76, 78. The central shaft
76 of the male rotor 14
has one end extending out of the housing 12. When the screw rotor device 10
operates as a
compressor, shaft 76 is rotated causing male rotor 14 to rotate. The male
rotor 14 causes the
female rotor 16 to counter-rotate through the gears 84, 86, and the helical
threads 34, 36 intermesh
with the helical grooves 38, 40.
As described above, the distal labyrinth seal 54 helps sealing between each of
the helical
threads 34, 36 on the male rotor 14 and the cylindrical bore 30 in the housing
12. Similarly, as
particularly illustrated in Figure 3, axial seals 88 may be formed in the
housing 12 along the length
of the cylindrical bore 32 to help sealing at the periphery of the female
rotor 16. As the male rotor
14 and female rotor 16 transition between meshing with each other and
respectively sealing around
the housing 12, a small gap 90 is formed between the male rotor 14, the female
rotor 16 and the
housing 12. The rotors 14, 16 fit in the housing 12 with close tolerances.
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As discussed above, the preferred embodiment of the screw rotor device 10 is
designed to
operate as a compressor. The screw rotor device 10 can be also be used as an
expander. When
acting as an expander, gas having a pressure higher than ambient pressure
enters the screw rotor
device 10 through the outlet port 20, valve 58 being optional. The pressure of
the gas forces
rotation of the male rotor 14 and the female rotor 16. As the gas expands into
the spaces 39, 41,
work is extracted through the end of shaft 76 that extends out of the housing
12. The pressure in
the spaces 39, 41 decreases as the gas moves towards the inlet port 18 and
exits into ambient
pressure at the inlet port 18. The, screw rotor device 10 can operate with a
gaseous working fluid
and may also be used as a pump for a liquid working fluid. For pumping
liquids, a valve may also
be used to prevent the fluid from backing into the rotor.
Figures 6A and 6B illustrate a detailed cross-sectional view of the helical
grooves and
helical threads from Figures 2A and 2B, respectively. These views illustrate
the differences
between an acme thread profile 92 and another feature of the present
invention, a buttress thread
profile 94. Between the minor diameter 50 and the major diameter 52 of the
female rotor, the
acme thread profile 92 of the helical groove 38 includes a concave line 96 and
a substantially
straight line 98 opposite therefrom. The buttress thread profile 94 also
includes a concave line 96
but is particularly defmed by a diagonal straight line 100. On the male rotor,
the acme thread 92
profile of the helical thread 34 is also between the major and minor diameters
and includes a pair
of opposing convex curves. In comparison, the buttress thread profile 94 has a
diagonal straight
line 102 that is parallel to and in close tolerance with the corresponding
diagonal straight line 100
in the helical groove 38. In the particular example illustrated by Figure 6B,
a convex curve 104 is
opposite the diagonal straight line 102.
Figures 7A and 7B particularly illustrate the screw rotor device 10 according
to several
aspects of the present invention, including the parallel diagonal straight
lines 100, 102 of the
buttress thread profile 94, phase-offset helical threads 34, 36, and the
single pitch design of the
male and female rotors 14, 16 within the housing 12. With regard to the
particular example
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illustrated by Figure 7B, the buttress thread profile 94 includes a concave
curve 104 opposite from
the diagonal straight line 102. It should be appreciated that the benefits of
the present invention
can be achieved with manufacturing tolerances, such as in the parallel
diagonal straight lines 100,
102. In particular, tolerances in the parallel diagonal straight lines 100,
102 may allow for a slight
radius of curvature between the diagonal lines and the major and minor
diameters and an
extremely slight divergence in the parallelism. It will be appreciated that
manufacturing tolerances
may vary depending on the type of material being used, such as metals,
ceramics, plastics, and
composites thereof, and depending on the manufacturing process, such as
machining, extruding,
casting, and combinations thereof.
Figure 8 illustrates an axial cross-sectional schematic view of another aspect
of the present
invention with respect to screw rotor device 110. As with screw rotor device
10 described above,
the screw rotor device 110 generally includes a housing 112 and a pair of
rotors 114, 116 having
an inlet port 118 and an outlet port 120. The inlet port 118 is preferably
located at the gearing end
122 of the housing 112, and the outlet port 120 is located at the opposite end
124 of the housing
112. The rotors 114, 116 intermesh as they respectively counter-rotate about a
pair of substantially
parallel axes 126, 1 28 within a pair of cylindrical bores 130, 132 extending
between ends 122,
124.
Generally, each one of the rotors 114, 116 has an identical number (N) of
helical threads,
and in the preferred embodiinent of this aspect of the present invention, each
one of the rotors 114,
116 has a pair of helical threads 134, 136. Eacl1 one of the helical threads
134, 136 preferably has
a convex side 138 and a concave side 140. As the rotors 114, 116 counter-
rotate with respect to
each other, the helical threads 134, 136 on one of the rotors 114 respectively
intermesh in phase
with the helical threads 134, 136 on the other rotor 116. In this manner, the
working fluid flows
through the inlet port 118 and into the screw rotor device 110 in the spaces
139, 141 bounded on
each side of the helical threads 134, 136, the cylindrical bores 130, 132, and
the ends 122, 124 of
the housing 112. The spaces 139, 141 are alternatively opened to and closed
off from the inlet port
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118 as the helical threads 134, 136 intermesh. As the rotors 114 continue to
counter-rotate, the
working fluid is positively displaced toward and through the outlet port 120.
The intermeshing rotors 114, 116 are preferably twin rotors, as described in
reference to
Figures 9 and 10. In particular, the rotors 114, 116 are twins in nature
because they have. an
identical concave/convex cross-sectional shape 142 in the plane perpendicular
to the axes of
rotation 126, 128. The rotors 114, 116 counter-rotate with each otiler and_
intermesh without
locking up because their tlu=eads 134, 136 have opposite-handed helix angles
144. The
concavelconvex shape 142 generally includes a major diameter arc 146, a minor
diameter arc 148,
and concave and convex curves between the major and minor diameter arcs 146,
148 (Figure 13A). The
concave and convex curves respectively correspond to the concave and convex
sides 140, 138 of
the helical threads 134, 136. The concave curve 140 on each one of the rotors
114, 116 is
preferably defined by the path of the major diameter arc 146 on the otlier one
of the rotors 114,
116, respectively, and the concave curve 140 preferably has a.continually
decreasing radius from
the radius of the major diameter 146 to the radius of the minor diameter 148.
As the rotors 114,
116 counter-rotate, the radius of the concave curve 140 on one of the rotors
114, 116 decreases
while the radius of the identical concave curve 140 on the other one of the
rotors 116, 114
respectively increases, thereby maintaining the helical threads 134, 136 in
closest proximity to
each other between the axes of rotation 126, 128. In the preferred embodiment
of this aspect of the
present invention, the major diameter of the rotors 114, 116 is approximately
twice as long as the
minor diameter of the rotors 114, 116.
According to the present invention and described in reference to Figures 11
and 12, the
tightest tolerances between the l-ielical threads 134, 136 can be maintained
by defining the line of
closest proximity therebetween according to a buttress thread shape 150. In
particular, the buttress
thread shape 150 includes parallel straight diagonal lines 152 that almost
span the entire length of
the housing 112, with only a slight gap 154 between the rotors 114, 116 and
the ends 122, 124 of
the housing 112. The buttress thread shape 150 also includes a pair of
juxtaposed concave lines
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156 between the parallel straight diagonal lines 152. Although it is possible
for the parallel
straight diagonal lines 152 to span the length of the housing 112, such a
design would create an
extremely shaip edge between the helical threads 134, 136 and the cylindrical
bores 130, 132. As
the rotors 114, 116 counter-rotate, a pressure differential is produced on
either side of the helical
threads 134, 136 and a sharp edge between the helical threads 134, 136 creates
a Venturi effect
that increases the leakage in the region between the helical threads 134, 136
and the cylindrical
bores 130, 132. Therefore, the buttress thread shape 150 also preferably
includes two pairs of
straight lines 158, 160 that are located between the parallel straight
diagonal lines152 and the
juxtaposed concave lines 156. The straight lines 158, 160 can be rather short
an still improve the
sealing between the helical threads 134, 136 and the cylindrical bores 130,
132. In the prefeired
embodiment of this aspect of the present invention, the parallel straight
diagonal lines 152 are
more than three times as long the straight lines 158, 160 combined. The
straight lines 158, 160 are
substantially parallel to the axes of rotation 126, 128 and are offset from
each other. Additionally,
the straight lines 158, 160 are preferably the same length.
Generaily, the concave curve 140 for each one of the rotors 114, 116 is
defined by the slope
162 of the parallel straight diagonal lines 152 and by the diameters 164, 166
and arc angles 168,
170 of the major and minor diameters 172, 174, respectively. In Figures 13A
and 13B, the arc
angles 168, 170 are increased. By increasing the arc angles 168, 170, the
length of the straight
lines 158, 160 and the parallel straight diagonal lines 152 are respectively
increased and decreased
according to the helix angle 144, thereby causing the slope 162 of the
parallel straight diagonal
lines 152 to change.
As particularly illustrated in Figure 8; the pair of rotors 114, 116 has a
respective central
shaft 176, 178 in each one of these embodiments. The shafts 176, 178 are
rotatably mounted
within the housing 112 tlirough bearings 180 and seals 182. The rotors 114,
116 are preferably
linked to each other through a pair of counter-rotating gears 184, 186 that
are respectively attached
to the shafts 176, 178. The central shaft 176 of one of the rotors 114 has one
end extending out of
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the housing 112. When the screw rotor device 110 operates as a compressor,
shaft 176 is rotated
causing the corresponding rotor 114 to rotate. The actuated rotor 114 causes
the other rotor 116 to
counter-rotate through the gears 184, 186, and the rotors 114,116.intermesh
with each other.
Although each one of the rotors 14, 16 has an identical number (N) of helical
threads, the
particular number of helical threads 134, 36 can vary. For exainple, Figures
14A, 14B and 14C
show rotors 114, 116 that each have three helical threads 188, 190, 192. _. As
in the preferred
embodiment for this aspect of the invention, these rotors 114, 116 also have a
buttress thread
profile 150. As illustrated in Figure 14C, the radius of the concave curve 140
on one of_the rotors
114, 116 decreases while the radius of the identical concave curve 140 on the
other one of the rotors
116, 114 respectively increases as the rotors 114, 116 counter-rotate, thereby
maintaining the
helical tlireads 134, 136 in closest proximity to each other between the axes
of rotation 126, 128.
For balancing each one of the rotors 14, 16 on their respective shafts 176,
178, it is preferable to
have multiple helical threads 134, 136, although it will be appreciated that a
single helical thread
can also be used.
In the preferred embodiment of the present invention, the screw rotor device
110 operates
as a screw compressor on a gaseous working fluid. When operating as a screw
compressor, the
screw rotor device 110 preferably includes a valve 194 in operative fluid
communication with the
outlet port 120. As particularly disclosed in U.S. Patent No. 6,599,112,
the valve 194 may be a pressure timing plate attached to and rotating
with one of the rotors. The valve 194 may alternatively be a reed valve
attached to the housing
112. It will also be appreciated that the valve 194 can be other types of
pressure-actuated and
mechanically-actuated valves. A computer control system (not shown) could be
used to control
the valve 194 with actuators based on inputs from sensors. Additionally, a
valve may also be used
in controlling the entry of fluid into the screw rotor device 110 through the
inlet port 118.
The screw rotor device 110 can be also be used as an expander. When acting as
an
expander, gas having a pressure higher than ambient pressure enters the screw
rotor device 110
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through the outlet port 120. A valve system may also be used in controlling
the expansion of the
gas through the screw rotor device 110. The pressure of the gas forces
rotation of the rotors 114,
116. As the gas expands into the alternating spaces 139, 141, work is
extracted through the end of
shaft 176 that extends out of the housing 112. The pressure in the spaces 139,
141 decreases as the
gas moves towards the inlet port 118 and exits into ambient pressure at the
inlet port 118. The
screw rotor device 110 can operate with a gaseous working fluid and may also
be used as a pump
for a liquid working fluid. For pumping liquids, a valve may also be used to
prevent the fluid from
backing into the rotor.
The present invention is generally directed toward screw rotor devices 110
having rotors
114, 116 with the identical number of threads (N), a buttress thread profile
150 and a length that is
either approximately equal to or less than a single pitch 196 of the helical
threads 134, 136. The
pitch of a screw is generally defined as the distance from any point on a
screw thread to a
corresponding point on the next thread, measured parallel to the axis and on
the same side of the
axis. Each embodiment of the screw rotor device 110 illustrated in Figures 8-
13 has a pair of
helical threads 134, 136. Therefore, a 180 helical twist of the helical
threads 134, 136 produces a
single pitch of the helical threads 134, 136. In comparison, the embodiment of
the screw rotor
device 10 illustrated in Figures 14A, 14B and 14C has three helical threads
188, 190, 192.
Therefore, a 120 helical twist of the helical threads 188, 190, 192 produces
a single pitch of the
helical threads 188, 190, 192. In general, the helical twist required to
provide the single pitch is
merely defined by the number of helical threads according to equation (3)
above.
In each of the embodiments illustrated in Figures 8-14, the rotors 114, 116
are twins,
having an identical concave/convex cross-sectional shape 142 in the plane
perpendicular to the
axes of rotation 126, 128. However, the screw rotor device 110 may also have
rotors 114, 116
with that are not twins although the rotors 114, 116 may still have the
identical number of threads
(N), a buttress thread profile 150, and a length that is no greater than
approximately a single pitch
196. As discussed in detail above, Figure 7A illustrates an example of one
such design in which
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one of the rotors 14, 16 has a pair of helical threads with different
concave/convex cross-sectional
shapes 106, 107. As illustrated in Figure 7B, the different concave/convex
cross-sectional shapes
106, 107 result in different lengthwise profiles 108, 109 for the rotors 14,
16. In comparison, the
lengthwise profile in each of the other embodiments is the same shape, merely
being up-side-down
with respect to each other.
Of course, it will be appreciated that altllough the length of the screw rotor
device 110 is
limited to approximately a single pitch 196 of the helical threads 134, 136,
the pitch length can be
changed by altering the helix angle 144 of the helical threads 134, 136. The
pitch length increases
as the helix angle 144 steepens. Additionally, for rotors having given
diameters, the helix angle
144 will steepen as the number of thread increases. For exainple, the three-
thread embodiment
illustrated in Figure 14A has the same major and minor diameters as the two-
thread embodiment
illustrated in Figure 9, and these embodiments also have approximately the
same arc angle for the
major and minor diaineters. Therefore, although both of these embodiments have
a buttress thread
shape 150 with approximately the same slope 162, the rotors 114, 116 in these
embodiments do
not have the same helix angle 144 because the three-thread embodiment has a
180 helical twist
whereas the two-thread embodiment only has a 120 helical twist. Therefore,
the three-thread
embodiment has a steeper helix angle 144 than the two-thread embodiment.
As discussed above, the diameters 164, 166 and arc angles 168, 170 of the
major and
minor diameters 172, 174, respectively, are also variable. It should also be
appreciated that more
than two rotors can also be used according to the present invention and that
the rotors may have
different major and minor diameters. Additionally, it should be appreciated
that the axes of the
rotors do not necessarily need to be parallel with respect to each other,
although it is preferable for
the axes to be in the same plane. Therefore, according to the several aspects
of the present
invention as set forth in the following claims and described herein, the screw
rotor device 110 can
have alternative designs.
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The foregoing embodiments illustrate the screw rotor device 10, 110 according
to several
aspects of the present invention. The rotors 14, 16, 114, 116 generally fit
within the housing 12,
112 according to close tolerances, such as the gap 90, 154 discussed above,
and it should be
appreciated that the benefits of the present invention can be achieved within
manufacturing
tolerances, such as in the parallel diagonal straight lines 152 of the
buttress thread profile 150. In
particular, tolerances in the parallel diagonal straight lines 152 may allow
for a slight radius of
curvature between the diagonal lines and the major and minor diameters and an
extremely slight
divergence in the parallelism. It will be appreciated that manufacturing
tolerances may vary
depending on the type of material being used, such as metals, ceramics,
plastics, and composites
thereof, and depending on the manufacturing process, such as machining,
extruding, casting, and
combinations thereof.
From the detailed description of each of the embodiments above, it will be
appreciated that
the cross-sectional shape of the rotors can be different or identical, i.e.,
twin rotors. Regardless,
the screw rotor device 10, 110 of the present invention has an identical
number of threads (N) and
the twisting of the cross-sectional shape along the respective rotor axes 26,
28, 126, 128 results in
a helical shape for each rotor that intermeshes with the helical shape of the
other rotor.
Accordingly, the rotors can generally be referred to as having intermeshing
helical element pairs,
i.e., 34 & 38, 36 & 40,134 & 134, 136 & 136.
As discussed in detail above, the helical element pairs can be, in the
alternative or in any
combination, a phase-offset thread and a corresponding phase-offset groove, a
pair of single-pitch
buttress threads, or a pair of single-pitch concave/convex threads. In
particular, the phase-offset
helical threads on the male rotor mesh with the identical number of
corresponding phase-offset
helical grooves on the female rotor. In one aspect of the phase-offset helical
threads, the helical
groove can have a cut-back concave profile that meshes with a corresponding
cut-in convex profile
of the helical thread. The cut-back concave profile corresponds with a helical
groove having a
radially narrowing axial width at the periphery of the female rotor. In
another aspect of the phase-
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offset helical threads and corresponding grooves, these helical element pairs
can have the buttress
thread profile, i.e., a diagonal line between the intermeshing rotors. The
concave portion of the
concave-convex thread is formed by a path of the major diameter arc on the
thread other of the
intermeshing rotors, whereas the convex curve for the intermeshing rotors can
be defined by a
slope of the diagonal lines along with the diameter and arc angle of the
rotors' major diameter and
minor diameter arcs. Additionally, the maximum length of the rotors can be
limited to a single
pitch of the helical element pairs.
The cross-section of the phase-offset helical thread, in any plane
perpendicular to one of
the pair of axes, has the tooth and toothless sector. The sector is subtended
by the arc angle that is
proportionally greater than the tooth's arc angle, i.e., by the phase-offset
multiplier. The profile of
the tooth is a minor diameter arc and a tooth segment radially extending to a
major diameter arc in
close tolerance with the housing. The phase-offset multiplier is at least two.
The cross-section of the pair of single-pitch buttress threads, in a
lengthwise cross-section
of the pair of intermeshing rotors by a plane extending between the pair of
axes, has parallel
straight diagonal lines and a pair of opposing concave lines. The length of
the intermeshing rotors
is approximately equal to a single pitch of the single-pitch buttress threads.
The cross-section of said single-pitch concave/convex threads, in a plane
perpendicular to
one of the pair of axes, has a major diameter arc, a minor diameter arc, and
both a concave curve
and a convex curve tlierebetween. The concave curve on each one of the
intermeshing rotors is
defmed by a path of the major diameter arc on the other of the intermeshing
rotors, and the convex
curve for each of the intermeshing rotors is defined by several features of
the rotors, including the
slope of said parallel straight diagonal lines and the diameter and arc angle
of the major diameter
arc and the minor diameter arc. As with the single-pitch buttress thread, the
length of the
intermeshing rotors is approximately equal to a single pitch of the single-
pitch concave/convex
threads.
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In view of the foregoing, it will be seen that the several advantages of the
invention are
achieved and attained. The embodiments were chosen and described in order to
best explain the
principles of the invention and its practical application to thereby enable
others skilled in the art to
best utilize the invention in various embodiments and with various
modifications as are suited to
the particular use contemplated. As various modifications could be made in the
constructions and
methods herein described and illustrated without departing from the scope of
the invention, it is
intended that all matter contained in the foregoing description or shown in
the accompanying
drawings shall be interpreted as illustrative rather than limiting. Thus, the
breadth and scope of the
present invention should not be limited by any of the above-described
exemplary embodiments,
but should be defined only in accordance with the following claims appended
hereto and their
equivalents.
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