PAIR OF CO-OPERATING SCREW ROTORS

Abstract

A pair of co-operating screw rotors including a male rotor and a female rotor. The male rotor and the female rotor have helically extending lobes and intermediate grooves configured to intermesh with one another. Each groove of the female rotor has a first flank including at least three concave sections. A first section includes or is disposed immediately adjacent the radially innermost point of the groove. A second section is shaped as a circular arc with a radius having its center located outside the pitch circle. A third section is shaped as a circular arc with a radius having its center located outside the pitch circle. The radius of the third section is greater than the radius of the second section, which is greater than the radial distance between a pitch circle of the female rotor and the radially innermost point of the groove.

Claims

1. A pair of co-operating screw rotors, comprising: a male rotor having helically extending lobes and intermediate grooves; and a female rotor having: helically extending lobes and intermediate grooves which are configured to intermesh with the helically extending lobes and intermediate grooves of said male rotor; a pitch radius (R.sub.FP) defining a pitch circle (CFP), each groove of the female rotor having a first flank comprising at least three concave sections, and a first section comprises the radially innermost point of said groove, a second section is shaped as a circular arc with a radius (R2) having its center located outside the pitch circle, and a third section is shaped as a circular arc with a radius (R3) having its center located outside the pitch circle, said radius (R3) of the third section being greater than said radius (R2) of the second section which is greater than the radial distance (R) between the pitch circle and said radially innermost point of said groove.

2. The pair of co-operating screw rotors according to claim 1, wherein said first section is shaped as a circular arc with a radius (R1) having its center located on the pitch circle.

3. The pair of co-operating screw rotors according to claim 1, wherein each lobe of the male rotor has a first lobe flank comprising a convex section which is shaped as a circular arc with a radius having its center located on or inside the pitch circle of the male rotor, and wherein said first section of said female rotor is generated by said convex section of said male rotor.

4. The pair of co-operating screw rotors according to claim 1, wherein the second section is shaped as a circular arc with a radius (R2) having its center located on a straight line extending radially from an end point of said first section along the normal direction of said first section.

5. The pair of co-operating screw rotors according to claim 1, wherein the third section is shaped as a circular arc with a radius (R3) having its center located on a straight line extending radially from an end point of said second section along the normal direction of said second section.

6. The pair of co-operating screw rotors according to claim 2, wherein said center of said first section is defined by the crossing of the pitch circle and a straight line traversing the center of the rotor and the radially innermost point of said groove.

7. The pair of co-operating screw rotors according to claim 6, wherein said radius of said first section corresponds to the radial distance between the pitch circle and said radially innermost point of said groove.

8. A pair of co-operating screw rotors according to claim 1, wherein said sections are formed immediately adjacent to each other.

9. The pair of co-operating screw rotors according to claim 1, wherein said sections are consecutively disposed.

10. The pair of co-operating screw rotors according to claim 1, wherein said radius (R2) of the second section is 1.25 to 1.75 times the radial distance (R) between the pitch circle and said radially innermost point of said groove, and wherein said radius (R3) of the third section is 2 to 3 times the radial distance (R) between the pitch circle and said radially innermost point of said groove.

11. The pair of co-operating screw rotors according to claim 2, wherein each lobe of the male rotor has a first lobe flank comprising a substantially convex section which is generated at least partly by said at least three concave sections of said female rotor.

12. The pair of co-operating screw rotors according to, wherein each lobe of the male rotor has a first lobe flank comprising a substantially convex section which is generated at least partly by said second and third sections of said female rotor.

13. The pair of co-operating screw rotors according to claim 1, wherein each groove of the female rotor has a second flank comprising a substantially concave section, each lobe of the male rotor has a second lobe flank comprising a substantially convex section which is generated by said concave section of said female rotor.

14. The pair of co-operating screw rotors according to claim 2, wherein the second section is shaped as a circular arc with a radius (R2) having its center located on a straight line extending radially from an end point of said first section along the normal direction of said first section.

15. The pair of co-operating screw rotors according to claim 3, wherein the second section is shaped as a circular arc with a radius (R2) having its center located on a straight line extending radially from an end point of said first section along the normal direction of said first section.

16. The pair of co-operating screw rotors according to claim 2, wherein said radius (R2) of the second section is 1.25 to 1.75 times the radial distance (R) between the pitch circle and said radially innermost point of said groove, and wherein said radius (R3) of the third section is 2 to 3 times the radial distance (R) between the pitch circle and said radially innermost point of said groove.

17. The pair of co-operating screw rotors according to claim 3, wherein said radius (R2) of the second section is 1.25 to 1.75 times the radial distance (R) between the pitch circle and said radially innermost point of said groove, and wherein said radius (R3) of the third section is 2 to 3 times the radial distance (R) between the pitch circle and said radially innermost point of said groove.

18. The pair of co-operating screw rotors according to claim 17, wherein each groove of the female rotor has a second flank comprising a substantially concave section, each lobe of the male rotor has a second lobe flank comprising a substantially convex section which is generated by said concave section of said female rotor.

19. The pair of co-operating screw rotors according to claim 3, wherein each groove of the female rotor has a second flank comprising a substantially concave section, each lobe of the male rotor has a second lobe flank comprising a substantially convex section which is generated by said concave section of said female rotor.

20. The pair of co-operating screw rotors according to claim 4, wherein each groove of the female rotor has a second flank comprising a substantially concave section, each lobe of the male rotor has a second lobe flank comprising a substantially convex section which is generated by said concave section of said female rotor.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing currently preferred embodiment(s) of the invention, wherein

[0028] FIGS. 1-3 illustrate a rotary screw compressor according to generally known technique, and the function principle is explained in relation thereto,

[0029] FIG. 4 shows a pair of screw rotors of the known G-profile type, and

[0030] FIG. 5 shows a portion of a female rotor of an embodiment of a pair of screw rotors according to the invention, and

[0031] FIG. 6 shows portions of a female and a male rotor of another embodiment of a pair of screw rotors according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0032] In the following description, known screw rotors according to the prior art and also embodiments of the present invention are described.

[0033] FIGS. 1-3 illustrate a rotary screw compressor according to generally known technique. The compressor includes a pair of meshing screw rotors 1, 2 operating in a working space limited by two end walls 3, 4 and a barrel wall 5 extending between these, which barrel wall 5 has an internal shape substantially corresponding to that of two intersecting cylinders as can be seen in FIG. 2.

[0034] Each rotor 1, 2 has a plurality of lobes, and intermediate grooves extending helically along the entire rotor. One rotor 1 is of the male rotor type with the major part of each lobe located outside the pitch circle and the other rotor is of the female rotor type with the major part of each lobe located inside the pitch circle. The female rotor normally has more lobes than the male rotor 1, and a common lobe combination is 4+6. Low pressure air or gas is admitted into the working space of the compressor through an inlet port 8, is then compressed in the chevron-shaped working chambers formed between the rotors and the walls of the working space. Each chamber travels to the right in FIG. 1 as the rotors rotate, and the volume of a working chamber will continuously decrease during the later stage of its cycle after communication with the inlet port 8 has been cut off. Thereby the air or gas will be compressed, and the compressed air or gas leaves the compressor through an outlet port 9. The internal pressure ratio will be determined by the internal volume ratio, i.e. the relation between the volume of a working chamber immediately after its communication with the inlet port 8 has been cut off and the volume of a working chamber when it starts to communicate with the outlet port 9.

[0035] The compression cycle is schematically illustrated in FIG. 3, which shows the barrel wall developed in a plane, the vertical lines representing the two cusps, i.e. the lines along which the cylinders forming the working space intersect. The inclined lines represent the sealing lines established between the lobe tops and the barrel wall, which lines travel in the direction of the arrow C as the rotors rotate. The shaded area A represents a working chamber just after it has been cut off from the inlet port 8 and the shaded area B a working chamber that has started to open towards the outlet port 9. As can be seen the volume of each chamber increases during the filling phase when the chamber communicates with the inlet port 8 and thereafter decreases.

[0036] In FIG. 4 a pair of screw rotors of the known G-profile type is shown. The rotors rotate as indicated by the arrows, the male rotor being the driving rotor. The leading flank of the male rotor lobe has a profile segment 11 being a circular arc. On the trailing flank of the female rotor lobe, i.e. the leading flank of the female rotor groove, there is a corresponding circular arc flank segment 10 co-operating with the circular arc flank segment 11 of the male rotor lobe 7 so that a contact band is created through which torque is transmitted from the male rotor 1 to the female rotor 2. In FIG. 4 the mesh position, when the circular arc segments 10, 11 contact each other, is shown for male drive. As can be seen in the figure, the tangent of the leading flank of the female rotor groove at the pitch circle forms a very small angle α.sub.1 with a radial line drawn through the center of the rotor. The corresponding angle α.sub.2 of the trailing flank of the groove is also very small. Thus, the profile has a closed character, making it difficult to manufacture in manufacturing tools, requiring substantially parallel edges of the cutting tool at the outer portion thereof. Such a shape of the cutter induces a high wear thereof, and a high amount of tool material has to be ground away during each re-sharpening. Since the number of possible re-sharpenings is limited, tools costs will be a significant part of the final cost of the rotor.

[0037] FIG. 5 shows a portion of a female rotor of an embodiment of a pair of screw rotors according to the invention. The female rotor rotates as indicated by the arrow, being driven by a male rotor (not shown). In the figure, two helically extending lobes and an intermediate groove is shown. The female rotor has a pitch radius R.sub.FP defining a pitch circle C.sub.FP relative a center O.sub.F of the female rotor. The illustrated groove has a first or leading flank comprising at least three concave sections 12, 13, 14. The sections 12, 13, 14 are formed immediately adjacent to each other and consecutively. A first section 12 comprises the radially innermost point 16 of the groove. The first section is shaped as a circular arc with a radius R.sub.1 having its center O.sub.1 located on the pitch circle. R.sub.1 equals the radial distance R between the pitch circle and the radially innermost point of the groove. The center O.sub.1 is defined by the crossing of the pitch circle and a straight line traversing the center of the rotor and the radially innermost point 16 of the groove. A second section 13 is shaped as a circular arc with a radius R.sub.2 having its center O.sub.2 located outside the pitch circle. The center 0.sub.2 is located on a straight line extending from the end point 17 of the first section through O.sub.1, at a distance R.sub.2 from the groove. The straight line extending between the end point 17 and O.sub.1 may also be described as the limiting line of the first section. A third section 14 is shaped as a circular arc with a radius R.sub.3 having its center O.sub.3 located outside the pitch circle. The center O.sub.3 is located on a straight line extending from the end point 18 of the second section through O.sub.2, at a distance R.sub.3 from the groove. The straight line extending between the end point 18 and O.sub.2 may also be described as the limiting line of the second section. As can be seen in the figure, the radius of the third section is greater than the radius of the second section, which is greater than the radius of the first section. Advantageously, the radius of the second section is 1.25 to 1.75 times the radius of the first section, and the radius of the third section is 2 to 3 times the radius of the first section. The illustrated groove also has a second flank opposite the first flank which comprises a convex section 15. The section 15 may be generated by a corresponding section of the male rotor.

[0038] FIG. 6 shows portions of a male and a female rotor of another embodiment of a pair of screw rotors according to the invention. In the figure, a portion of a female rotor with two helically extending lobes and an intermediate groove, and a portion of a male rotor with two helically extending lobes and an intermediate groove are shown. The male and female rotors are illustrated at a distance from each other, it is however understood that in use, the two rotors are essentially in contact with each other at least at one point, i.e. has a very tight play to avoid leakage. The rotors rotate as indicated by the arrow, the male rotor being the driving rotor.

[0039] The female rotor has a pitch radius R.sub.FP defining a pitch circle C.sub.FP. The illustrated groove has a first or leading flank comprising at least three concave sections 112, 113, 114. The sections 112, 113, 114 are formed immediately adjacent to each other and consecutively. A first section 112 comprises the radially innermost point 116 of the groove. A second section 113 is shaped as a circular arc with a radius R.sub.2 having its center O.sub.2 located outside the pitch circle. The center O.sub.2 is located on a straight line extending from the end point 117 of the first section along the normal direction of the first section, at a distance R.sub.2 from the groove. A third section 114 is shaped as a circular arc with a radius R.sub.3 having its center O.sub.3 located outside the pitch circle. The center O.sub.3 is located on a straight line extending from the end point 118 of the second section through O.sub.2, at a distance R.sub.3 from the groove. As can be seen in the figure, the radius of the third section is greater than the radius of the second section, which is greater than the radial distance R between the pitch circle and the radially innermost point 116 of said groove. Advantageously, R.sub.2 is 1.25 to 1.75 times R, and R.sub.3 is 2 to 3 times R.

[0040] FIG. 6 also shows a first or leading flank of a lobe of the male rotor, which first or leading flank comprises at least three essentially convex sections 119, 120, 121. The sections 119, 120, 121 are formed immediately adjacent to each other and consecutively. A first section 119 is shaped as a circular arc with a radius R.sub.1 having its center coinciding with the center O.sub.M of the male rotor, thus inside of the pitch circle C.sub.MP of the male rotor. The first section 112 of the female rotor is generated by the first section 119 of the male rotor, i.e. is the envelope of the first section of the male rotor, while the second and third sections 120, 121 of the male rotor are generated by the second and third section 113, 144 of the female rotor, respectively.

[0041] Although exemplary embodiments of the present invention have been shown and described, it will be apparent to the person skilled in the art that a number of changes and modifications may be made. It is understood that the above description of the invention and the drawings are to be regarded as non-limiting examples thereof and that the scope of the invention is defined by the patent claims.