CA2504474C

CA2504474C - Improved screw rotor device

Info

Publication number: CA2504474C
Application number: CA002504474A
Authority: CA
Inventors: Charles K. Heizer
Original assignee: IMPERIAL RESEARCH LLC
Current assignee: IMPERIAL RESEARCH LLC
Priority date: 2002-10-29
Filing date: 2003-10-28
Publication date: 2008-09-09
Anticipated expiration: 2023-10-28
Also published as: MXPA05004582A; WO2004040133A3; CA2504474A1; EP1556610A2; WO2004040133A2; AU2003285043A1

Abstract

A screw rotor device has a pair of intermeshing rotors (14, 16) that are rotatably mounted in a housing around a pair of axes (26, 28). The cross-sectional shape of the rotors can be identical, i.e., twin rotors, or the rotors may have different cross-sectional shapes, i.e., a male and female.
Generally, the screw rotor has an identical number of threads 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 other rotor (14, 16).
Accordingly, the rotors (14, 16) can be referred to as having intermeshing helical element pairs (34, 36). In several 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 (50), or a pair of single-pitch concave/convex threads.

Description

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 respectively 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.

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 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.

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 .

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 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 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 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 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.

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.

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 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 =

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 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 ~

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 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 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.

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-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.

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.

Claims

What is Claimed Is:

1. A screw rotor device for positive displacement of a working fluid, comprising:
a housing having an inlet port at a first end and an outlet port at a second end and a pair of cylindrical bores extending therebetween;
a pair of intermeshing rotors rotatably mounted about a respective pair of axes between said first end and said second end of said housing, wherein said pair of rotors have an identical number of intermeshing helical element pairs, wherein said helical element pairs are comprised of a phase-offset thread and a corresponding phase-offset groove in the form of a pair of buttress threads;
wherein a cross-section of said buttress threads, in any plane perpendicular to one of said pair of axes, comprises a tooth and a toothless sector, said tooth being subtended by a first arc angle with respect to said axis and said sector having a second arc angle proportionally greater than said first arc angle by a phase-offset multiplier, said tooth having a profile comprising a minor diameter arc and a tooth segment radially extending to a major diameter arc in close tolerance with said housing, and wherein said phase-offset multiplier is at least two for any crosssection taken in any said perpendicular plane; and wherein a cross-section of said pair of buttress threads, in a lengthwise cross-section of said pair of intermeshing rotors by a plane extending between said pair of axes, is comprised of parallel straight diagonal lines and a pair of opposing concave lines.

2. The screw rotor device according to claim 1, wherein said buttress threads are further comprised of a concave/convex thread cross-section.

3. The screw rotor device according to claim 2, wherein said concave/convex thread cross-section, in a plane perpendicular to one of said pair of axes, is comprised of a major diameter arc, a minor diameter arc, a concave curve between said major diameter arc and said minor diameter arc and a convex curve between said minor diameter arc and said major diameter arc, wherein said concave curve on each one of said pair of intermeshing rotors is defined by a path of said major diameter arc on the other of said pair of intermeshing rotors and said convex curve for each of said pair of intermeshing rotors is defined by a slope of said parallel straight diagonal lines and by a diameter and arc angle of said major diameter arc and said minor diameter arc, and wherein a length of said pair of intermeshing rotors is approximately equal to a single pitch of said single-pitch concave/convex threads.

4. The screw rotor device according to claim 2, wherein a length of said pair of intermeshing rotors is approximately equal to a single pitch of said phase-offset thread.

5. The screw rotor device according to claim 1, wherein said buttress threads are further comprised of a concave/convex cross-section in a plane perpendicular to one of said pair of axes, wherein said concave/convex cross-section is comprised of a major diameter arc, a minor diameter arc, a concave curve between said major diameter arc and said minor diameter arc and a convex curve between said minor diameter arc and said major diameter arc.

6. The screw rotor device according to claim 5, wherein said concave curve on each one of said pair of intermeshing rotors is defined by a path of said major diameter arc on the other of said pair of intermeshing rotors and said convex curve for each of said pair of intermeshing rotors is defined by a slope of said parallel straight diagonal lines and by a diameter and arc angle of said major diameter arc and said minor diameter arc, and wherein a length of said pair of intermeshing rotors is approximately equal to a single pitch of said single-pitch concave/convex threads.

7. The screw rotor device according to claim 5, wherein a length of said pair of intermeshing rotors is approximately equal to a single pitch of said phase-offset thread.

8. The screw rotor device according to claim 1, wherein a length of said pair of intermeshing rotors is approximately equal to a single pitch of said phase-offset thread.

9. The screw rotor device according to claim 8, wherein said buttress threads are further comprised of a concave/convex cross-section and wherein a length of said pair of intermeshing rotors is approximately equal to a single pitch of said phase-offset thread.

10. A screw rotor device for positive displacement of a working fluid, comprising:
a housing having an inlet port at a first end and an outlet port at a second end and a pair of cylindrical bores extending therebetween;
a pair of intermeshing rotors rotatably mounted about a respective pair of axes between said first end and said second end of said housing, wherein said pair of rotors have an identical number of helical threads and a length approximately equal to a single pitch of said helical threads, said helical threads having a buttress thread shape in a lengthwise crosssection of said pair of rotors in a plane extending between said pair of axes, wherein said buttress thread shape is comprised of parallel straight diagonal lines and a pair of opposing concave lines.

11. The screw rotor device according to claim 10, wherein said buttress thread shape is further comprised of a first pair of straight lines substantially parallel to said pair of axes and located between said parallel straight diagonal lines and said opposing concave lines.

12. The screw rotor device according to claim 11, wherein said first pair of straight lines are approximately the same length for each rotor and said parallel straight diagonal lines are more than three times as long as said first pair of straight lines.

13. The screw rotor device according to claim 12, wherein said buttress thread shape is further comprised of a second pair of straight lines substantially parallel to and offset from said first pair of straight lines, wherein said second pair of straight lines are substantially the same length as said first pair of straight lines.

14. The screw rotor device according to claim 12, further comprising a second pair of straight line substantially parallel to and offset from said first pair of straight line, wherein said second pair of straight lines have a different length from said first pair of straight lines.

15. The screw rotor device according to claim 10, wherein each one of said pair of rotors has a concave/convex cross-sectional shape in a plane perpendicular to said pair of axes, wherein said concave/convex cross-sectional shape is comprised of a major diameter arc, a minor diameter arc, a concave curve between said major diameter arc and said minor diameter arc and a convex curve between said minor diameter arc and said major diameter arc, wherein said concave curve on each one of said pair of rotors is defined by a path of said major diameter arc on the other of said pair of rotors and said convex curve for each of said pair of rotors is defined by a slope of said parallel straight diagonal lines and by a diameter and arc angle of said major diameter arc and said minor diameter arc.

16. The screw rotor device according to claim 15, wherein said concave/convex crosssectional shape is identical for said pair of rotors.

17. The screw rotor device according to claim 10, further comprising a valve in fluid communication with said outlet port, wherein said pair of rotors confine the working fluid to a space within said housing that is in fluid communication with said outlet port.

18. The screw rotor device according to claim 10, wherein each one of said helical threads has a helical twist approximately equal to 360°/N, where N is a number of helical threads for either one of said pair of rotors.

19. A screw rotor device for positive displacement of a working fluid, comprising:
a housing having an inlet port at a first end and an outlet port at a second end and a pair of cylindrical bores extending therebetween;
a first rotor rotatably mounted about a first axis between said first end and said second end of said housing, said first rotor having at least one helical thread with a first helix angle and a first cross-sectional shape in a plane perpendicular to said first axis;
a second rotor rotatably mounted about a second axis between said first end and said second end of said housing, said second rotor having at least one helical thread with a second helix angle opposite from said first helix angle and a second cross-sectional shape in a plane perpendicular to said second axis; and wherein said helical threads of said first rotor and said second rotor intermesh in a counterrotating manner, wherein said first rotor has an identical number of helical threads as said second rotor and wherein said first rotor and said second rotor have a length approximately equal to a single pitch of said helical threads, said helical threads having a buttress thread shape in a lengthwise cross-section of said first rotor and said second rotor in a plane extending between said first axis and said second axis, wherein said buttress thread shape is comprised of parallel straight diagonal lines and a pair of opposing concave lines.

20. The screw rotor device according to claim 19, wherein said buttress thread shape is further comprised of a first pair of straight lines substantially parallel to said first axis and said second axis and located between said parallel straight diagonal lines and said opposing concave lines.

21. The screw rotor device according to claim 20, wherein said buttress thread shape is further comprised of a second pair of straight lines substantially parallel to and offset from said first pair of straight lines, wherein said first pair of straight lines are approximately the same length for each rotor and said second pair of straight lines are approximately the same length as the first pair of straight lines.

22. The screw rotor device according to claim 19, wherein said first crosssectional shape and said second cross-sectional shape are each comprised of a major diameter arc, a minor diameter arc, a concave curve between said major diameter arc and said minor diameter arc, and a convex curve between said minor diameter arc and said major diameter arc.

23. The screw rotor device according to claim 22, wherein said first crosssectional shape and said second cross-sectional shape are identical and wherein said parallel straight diagonal lines comprise at least one-third of said length of said first rotor and said second rotor.

24. The screw rotor device according to claim 19, wherein each of said first rotor and said second rotor further comprises a plurality of helical threads, each one of said helical threads having a helical twist approximately equal to 360°/N, where N is the number of helical threads.

25. The screw rotor device according to claim 19, further comprising a valve in fluid communication with said outlet port, wherein said first rotor and said second rotor confine the working fluid to a space within said housing that is in fluid communication with said outlet port.

26. A screw rotor device for positive displacement of a working fluid, comprising:
a housing having an inlet port at a first end and an outlet port at a second end and a pair of cylindrical bores extending therebetween;
a pair of intermeshing rotors rotatably mounted about a respective pair of axes between said first end and said second end of said housing, wherein each one of said pair of rotors has an identical number of helical threads and has a length less than approximately a single pitch of said helical threads, said helical threads having a buttress thread shape in a lengthwise cross-section of said pair of rotors in a plane extending between said pair of axes, wherein said buttress thread shape is comprised of parallel straight diagonal lines, a pair of opposing concave lines, a first pair of straight lines substantially parallel to said pair of axes and located between said parallel straight diagonal lines and said opposing concave lines, and a second pair of straight lines substantially parallel to and offset from said first pair of straight lines, wherein said second pair of straight lines are substantially the same length as said first pair of straight lines, and said parallel straight diagonal lines are more than three times as long as said first and second pair of straight lines combined, and wherein each one of said pair of rotors has a concave/convex cross-sectional shape in a plane perpendicular to said pair of axes.

27. The screw rotor device according to claim 26, wherein said concave/convex crosssectional shape is identical for said pair of rotors and is comprised of a major diameter arc, a minor diameter arc, a concave curve between said major diameter arc and said minor diameter arc and a convex curve between said minor diameter arc and said major diameter arc, wherein said concave curve on each one of said rotors is defined by a path of said major diameter arc on the other of said rotors and said convex curve has a continually decreasing radius from a radius of said major diameter arc to a radius of said minor diameter arc.

28. The screw rotor device according to claim 26, wherein each one of said rotors has a pair of helical threads and said length of said rotors is approximately equal to said single pitch of said helical threads.

29. The screw rotor device according to claim 26, wherein each one of said rotors has a major diameter and a minor diameter, said major diameter being approximately twice as long as said minor diameter.

30. A screw rotor device for positive displacement of a working fluid, comprising:
a housing having an inlet port at a first end and an outlet port at a second end and a pair of cylindrical bores extending therebetween;
a male rotor having at least one phase-offset helical thread and having a length approximately equal to a single pitch of said helical thread, wherein said male rotor is rotatably mounted about a first axis extending between said first end and said second end of said housing, wherein a cross-section of said phase-offset helical thread, in any plane perpendicular to said first axis, comprises a tooth and a toothless sector, said tooth being subtended by a first arc angle with respect to said axis and said sector having a second arc angle proportionally greater than said first arc angle by a phase-offset multiplier, said tooth having a profile comprising a minor diameter arc and a tooth segment radially extending to a major diameter arc in close tolerance with said housing; and a female rotor having at least one helical groove and having a length approximately equal to a single pitch of said helical groove, wherein said female rotor is rotatably mounted about a second axis and counter-rotates with respect to said male rotor and has a periphery in close tolerance with said housing, and wherein said helical groove intermeshes with said helical thread.

31. The screw rotor device according to claim 30, wherein said male rotor and female rotor have an identical number of helical threads and helical grooves, respectively, and wherein said helical threads and helical grooves form a buttress thread shape in a lengthwise cross-section of said male rotor and said female rotor in a plane extending between said first axis and said second axis, wherein said buttress thread shape is comprised of parallel straight diagonal lines and a pair of opposing lines.

32. The screw rotor device according to claim 31, wherein said buttress thread shape is bounded by a first pair of straight lines corresponding with said minor diameter of said male rotor and said major diameter of said female rotor.

33. The screw rotor device according to claim 32, wherein said buttress thread shape is further comprised of a second pair of straight lines corresponding with said major diameter of said male rotor and said minor diameter of said female rotor and located between said parallel straight diagonal lines and said pair of opposing lines.

34. The screw rotor device according to claim 30, wherein said phase-offset multiplier is identical for each cross-section taken in any said perpendicular plane.

35. A screw rotor device for positive displacement of a working fluid, comprising:
a housing having an inlet port at a first end and an outlet port at a second end and a pair of cylindrical bores extending therebetween;
a male rotor having at least one phase-offset helical thread, wherein said male rotor is rotatably mounted about a first axis extending between said first end and said second end of said housing, wherein a cross-section of said phase-offset helical thread, in any plane perpendicular to said first axis, comprises a tooth and a toothless sector, said tooth being subtended by a first arc angle with respect to said axis and said sector having a second arc angle proportionally greater than said first arc angle by a phase-offset multiplier, said tooth having a profile comprising a minor diameter arc and a tooth segment radially extending to a major diameter arc in close tolerance with said housing, and wherein said helical thread forms a first buttress thread shape in a lengthwise cross-section of said male rotor in a plane extending between said first axis and said second axis, wherein said first buttress thread shape is comprised of a first line extending from said minor diameter to said major diameter and a straight line extending diagonally away from said first line; and a female rotor having at least one helical groove, wherein said female rotor is rotatably mounted about a second axis and counter-rotates with respect to said male rotor and has a periphery in close tolerance with said housing, and wherein said helical groove intermeshes with said helical thread and forms a second buttress thread shape in a lengthwise cross-section of said female rotor in a plane extending between said first axis and said second axis, wherein said second buttress thread shape corresponds with said first buttress thread shape and is comprised of a second line extending between a minor diameter and a major diameter of said female rotor and a straight diagonal line extending away from said second line, wherein said straight diagonal line is substantially parallel to said straight line of said male rotor.

36. The screw rotor device according to claim 35, wherein said male rotor and said female rotor have a length approximately equal to a single pitch of said helical thread and said helical groove, respectively.

37. The screw rotor device according to claim 35, wherein said first and second buttress thread shapes are bounded by a first pair of straight lines corresponding with said minor diameter of said male rotor and said major diameter of said female rotor, respectively, and wherein said first and second buttress thread shapes are further comprised of a second pair of straight lines corresponding with said major diameter of said male rotor and said minor diameter of said female rotor, respectively.