GB2331127A

GB2331127A - Screw rotor set

Info

Publication number: GB2331127A
Application number: GB9824774A
Authority: GB
Inventors: Yoshinori Kurioka; Shoji Yoshimura
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 1997-11-11
Filing date: 1998-11-11
Publication date: 1999-05-12
Anticipated expiration: 2018-11-11
Also published as: GB9824774D0; GB2331127B; JPH11141479A

Abstract

In a screw rotor set comprising a female rotor F and a male rotor M engageable with the female rotor, the female rotor has a tooth flank defined by a curved line b<SP>~</SP>f comprising a curved line b<SP>~</SP>c forming a leading-side tooth flank generated by an arbitrarily determined tooth tip portion of a male rotor tooth TM, an arcuate curved line e<SP>~</SP>f which forms a seal S at a leading-side end portion of a tip outer periphery surface of the female rotor tooth TF, and a curved line c<SP>~</SP>e between the lines b<SP>~</SP>c and e<SP>~</SP>f and connected smoothly thereto. The line c<SP>~</SP>e is formed with a curvature which becomes larger continuously or discontinuously as the line e'f is approached. The male rotor M has a follow-up-side tooth flank generated by the curved line c<SP>~</SP>f.

Description

SCREW ROTOR BACKGROUND OF ; INVENEON Field of the Invention: The present invention relates to a screw rotor of, for example, a screw type compressor, a screw type vacuum pump, or a screw type expansion machine.

Description of the Prior Art: As to a screw compressor, a screw rotor of what is called a symmetric tooth profile was initially used, but a screw rotor of an asymmetric tooth profile superior in volume efficiency was developed by SRM Company, Sweden in 1965.

In Japanese Patent Publication No. 14723/76 is proposed such a screw rotor as shown in FIG. 8 in which a tooth space G and a sealing edge Sofa female rotor F lie inside a female rotor pitch circle, the sealing edge S being formed at an intersecting portion of both a straight line portion 32 following a generating curve 31 which defines a leading-side tooth flank of a female rotor tooth TF and a circular arc 33 forming a tip outer periphery surface of the female rotor tooth TF which constitutes a part of the rotor outer periphery and a remaining smooth circular protuberance 34 of the circular arc 33 joins a follow-up side concave curve 35 of the female rotor tooth TF.

In FIG. 8, M denotes a male rotor.

In the screw rotor shown in FIG. 8, in connection with the shape of the tip outer periphery surface of the female rotor tooth TF, by positioning the sealing edge S at an end of the tip outer periphery surface, the machining of the sealing edge S can be done without changing a cutter as is the case with machining of a tooth flank. However, when the sealing edge S is positioned at an end of the tip outer periphery surface, it is contiguous to the leading-side straight line portion 32 of the female rotor tooth TF while forming an angular portion rather than forming a gentle curve. Such a shape is not desirable for the machining of a tooth bottom of the male rotor M which mates with the female rotor F, as will be described below.

FIG. 9 illustrates a cutter 51 for machining the male rotor M and FIG. 10 is an enlarged diagram of a tip portion of the cutter 51 enclosed with a circle m.

FIG. 11 illustrates relative displacements of the cutter 51 with respect to the male rotor M at certain time intervals during machining of the male rotor M and FIG. 12 is an enlarged diagram of the portion enclosed with a circle IV in FIG. 11.

Alook at FIGS. 11 and 12 shows that, near the bottom of a tooth space of the male rotor M, loci of the cutter 51 are concentrated in the vicinity of the upper right-hand tooth flank, namely, in the vicinity of tooth flank A side. Thus, since the cutter 51 moves along the tooth flank A, an error in shape of the cutter 51 and an error in operation thereof are apt to be reflected on the tooth flank of the male rotor M on the side which is subjected to machining. Besides, since machining with a fine projection 52 at the tip of the cutter 51 continues for a long time in comparison with the other portion of the cutter 51, a thermal expansion and wear caused by an increase in temperature of the projection 52 become so much conspicuous. As a result, the dimensional accuracy ofthe portion machined by the projection 52, or the portion corresponding to the foregoing sealing edge S, is deteriorated.

As is seen from FIGS. 9 to 12, the tip projection 52 of the cutter 51 forms a relatively sharp angular portion. Therefore, the projection 52 is subject to wear during machining of the male rotor M. Also from this point there arises the problem of deteriorated dimensional accuracy of the male rotor Z SUMMARY OF TEE lNVEN'T'lON The present invention has been solved for eliminating the abovementioned problems and it is an object of the invention to provide a screw rotor of, for example, a screw compressor capable of improving the working efficiency and dimensional accuracy in rotor machining and durability of a cutter for rotor machining.

According to the present invention, for achieving the above-mentioned object, there is provided in the first respect thereof a screw rotor comprising a female rotor and a male rotor formed engageably with the female rotor the screw rotor including a leading-side tooth flank ofthe female rotor, the leading-side tooth flank being formed by a curved line which is generated by an arbitrarily determined tooth tip portion of the male rotor; an arcuate tooth tip outer periphery surface of the female rotor, with a sealing edge being formed at a leading-side end portion of the tooth tip outer periphery surface; a leading-side tip tooth flank of the female rotor interposed between the leading-side tooth flank of the female rotor and the tooth tip outer periphery surface ofthe female rotor, the leading-side tip tooth flank of the female rotor being connected smoothly to the leading-side tooth flank and the tooth tip outer periphery surface of the female rotor and formed at a curvature which becomes larger as the leading-side tip tooth flank approaches the tooth tip outer periphery surface of the female rotor; and a follow-up-side toot flank of the male rotor generated by a tooth flank which is determined by both the leading-side tip tooth flank and the tooth tip outer periphery surface of the female rotor.

In the second aspect of the present invention there is provided in combination with the screw rotor in the first aspect a screw rotor wherein the leading-side tip tooth flank of the female rotor is formed at a curvature which becomes larger continuously as the leading-side tip tooth flank approaches the tooth tip outer periphery surface of the female rotor.

In the third aspect of the present invention there is provided in combination with the screw rotor in the first aspect a screw rotor wherein the leading-side tip tooth flank of the female rotor is formed at a curvature which becomes larger discontinuously as the leading-side tip tooth flank approaches the tooth tip outer periphery surface of the female rotor.

In the fourth aspect of the present invention there is provided in combination with the screw rotor in the third aspect a screw rotor wherein the leading-side tip tooth flank of the female rotor is formed at two curvatures which become larger discontinuously as the leading-side tip tooth flank approaches the tooth tip outer periphery surface of the female rotor.

In the fifth aspect of the present invention there is provided in combination with the screw rotor in the fourth aspect a screw rotor wherein, at the portion of the smaller curvature of said two curvatures, the ratio of curvature radius to the radius of a pitch circle is 0.2 or more.

In the sixth aspect of the present invention there is provided in combination with the screw rotor in the first aspect a screw rotor wherein the sealing edge is formed outside a pitch circle of the female rotor.

BRIEF DESCRIPIION OF THE DRAWINGS FIG. 1 is a diagram showing a contour of a section perpendicular to the axis of an engaging portion of a screw rotor embodying the present invention; FIG. 2 is a diagram showing a cutter for machining a male rotor in the screw rotor shown in FIG. 1; FIG. 3 is an enlarged diagram of the portion enclosed with a circle I in FIG.

2; FIG. 4 is a diagram showing relative displacements of the cutter with respect to the male rotor at predetennined time intervals during machining of the male rotor with the cutter shown in FIG. 2; FIG. 5 is an enlarged diagram of the portion enclosed with a circle II in FIG. 4; FIG. 6 is a diagram showing a relation between a radius of curvature R1 (mm) and a blow-hole opening area Sb (mum2); FIG. 7 is a diagram showing a relation between a radius of curvature R2 (mm) and the blow-hole opening area Sb (mm2); FIG. 8 is a diagram showing a conventional screw rotor; FIG. 9 is a diagram showing a cutter for machining a male rotor in the screw rotor shown in FIG. 8; FIG. 10 is an enlarged diagram of the portion enclosed with a circle m in FIG. 9; FIG. 11 is a diagram showing relative displacements of the cutter with respect to the male rotor at predetermined time intervals during machining of the male rotor with the cutter shown in FIG. 9; and FIG. 12 is an enlarged diagram of the portion enclosed with a circle IV in FIG. 11.

DETAILED DESCRE'TION OF A PREFERRED EMBODIMENT An embodiment of the present invention will be described in detail hereinunder with reference to the accompanying drawings.

FIG. 1 illustrates a sectional shape of an engaging portion between a male rotor M and a female rotor F in a screw rotor of for example, a screw compressor according to an embodiment of the present invention.

During operation of the screw rotor, the male rotor M and the female rotor F rotate in the directions of arrows A and B, respectively. In FIG. 1, the lower side of a male rotor tooth TM is designated a leading-side tooth flank and the upper side thereof is designated a follow-up-side tooth flank. Likewise, the lower side of a female rotor tooth TF is designated a leading-side tooth flank and the upper side thereof is designated a follow-up-side tooth flank.

The leading-side tooth flank of the female rotor TF represented by a curved line be is a generating curve which is determined by an arbitrarily determined tooth tip portion of the male rotor tooth TM. The contour of a seal edge S of the leading-side end portion represented by a curved line e-f of the female rotor tooth TF is arcuate.

The curved lines but and ef are connected smoothly to a curved line formed with a radius of curvature which becomes larger continuously or discontinuously as the curved line approaches the curved line e~f. For example, the curved lines b--c and e--f are smoothly connected to a curved line c-e comprising a curved line c-d having a radius of curvature R1 and a curved line d-e having a radius of curvature R2 (R1 > R2) and connected smoothly to the curved line c~d, as shown in FIG. 1.

Although in the example shown in FIG. 1 there are adopted two curved lines of two different, discontinuously enlarging radii of curvature, the present invention is not limited thereto. There may be adopted more than two curved lines. Even a curved line which comprises an infinite number of curved lines and whose radius of curvature varies continuously, is also included in the present invention.

On the other hand, the male rotor M which mates with and engages the female rotor F has a follow-up-side tooth flank determined by the curved line c-f.

As noted previously, the sealing edge S is positioned at an intersection portion of the curved line be which defines the shape of the leading-side tooth flank of the female rotor tooth TF and the tip outer periphery surface of the female rotor tooth so it is possible for the same cutter to machine both the tooth flank of the female rotor F and the sealing edge S. As a result, it becomes possible to improve the working efficiency and dimensional accuracy in machining the female rotor F. Moreover, as will be described later with reference to the drawings, the dimensional accuracy in machining a groove portion at the tooth bottom of the male rotor M corresponding to the sealing edge S of the female rotor F, as well as the durability of the cutter which machines the tooth bottom portion, are improved.

Further, in the example shown in FIG. 1, the sealing edge S is formed outside a pitch circle PF of the female rotor F.

According to the above construction, the stroke volume can be increased in comparison with the conventional screw rotor, provided there is no change in center-to-center (OM, OF) distance of the male and female rotors. As a result, for the same stroke volume, it becomes possible to reduce the apparatus size.

FIG. 2 illustrates a cutter 21 for machining the male rotor M in the screw rotor embodying the invention and FIG. 3 is an enlarged diagram of the portion enclosed with a circle m in FIG. 2. At the tip outer periphery portion of the cutter 21 is formed a projection 22 for machining the groove portion at the tooth bottom of the male rotor M corresponding to the sealing edge S of the female rotor F. It is seen that the projection 22 shown in FIGS. 2 and 3 is less sharp than the projection 52 of the conventional cutter 51 shown in FIGS. 10 and 11. Therefore, the projection 22 shown in FIGS. 2 and 3 is larger in heat capacity, less damaged by overheating during machining of the sealing edge S and smaller in the amount of wear, with consequent improvement in dimensional accuracy of the male rotor M.

FIG. 4 shows relative displacements of the cutter 21 with respect to the male rotor M at predetermined time intervals during machining with the cutter 21 of the tooth bottom of the male rotor M, more particularly, the groove portion at the tooth bottom corresponding to the sealing edge S.

FIG. 5 is an enlarged diagram of the portion enclosed with a circle V in FIG. 4. From these figures it is seen that relative large motions with respect to the male rotor M are maintained during the above machining in the use of the cutter 21 shown in FIGS. 4 and 5 in comparison with the conventional cutter 51 used in such machining of the male rotor M as shown in FIGS. 11 and 12. In the above machining, therefore, an increase of temperature at the tip portion of the cutter 21 is suppressed, whereby the dimensional accuracy of the male rotor M is improved.

The radius of curvature R1 of the curved line c-d shown in FIG. 1 and the radius of curvature R2 of the curved line d-e shown in the same figure exert a great influence on the following parameters.

First, the radius of curvature R1 has a bearing on the blow-hole area which greatly influences the performance of a screw compressor or the like. The blow-hole means a gap which is formed among the paired intermeshing male and female rotor teeth TM, TF and the wall surface of a rotor chamber and which is peculiar to the associated screw compressor or the like. In the case of a compressor or a vacuum pump, an increase in the opening area of the gap, that is, an increase of the blow-hole area, results in that a compressed gas present in a tooth space of both rotors leaks back to an adjacent tooth space of a lower pressure, thus giving rise to a problem of deterioration in volume efficiency which is one of parameters representing the performance of the compressor or the like.

As an example, FIGS. 6 and 7 show the results of calculations conducted respectively on the relation between the radius of curvature R1 (mm) and the blowhole area Sb (mm2) and the relation between the radius of curvature R2 (mm) and the blow-hole area Sb (mm2). FIGS. 6 and 7 are of the case where the radius of a pitch circle is 135 mm.

The blow-hole area Sb is greatly dependent on the radius of curvature R1 and it decreases with increase of the curvature radius R1. On the other hand, the blow-hole area Sb proves to be little dependent on the radius of curvature R2.

Thus, it is desirable that the radius of curvature R1 be set large. As is seen from FIG. 6, it is desirable that the ratio of curvature radius to the pitch circle radius be set at 0.2 or more.

Next, as to the radius of curvature R2, it is seen from FIG. 5 that as the curvature radius R2 decreases, relative large motions of the cutter 21 with respect to the male rotor M are ensured and the cutting point continues to shift during machining of a tooth space of the male rotor M, especially the bottom portion thereo Thus, the wear of the cutter 21 and an increase of temperature at the tip of the cutter are not concentrated on one position, nor is there any convergence or accumulation of manufacturing errors in the manufacture of the male rotor M.

Consequently, it becomes easy to ensure a high dimensional accuracy.

Also as to the shape of the cutter 21, as shown in FIG. 3, the projection 22 is roundish outwards as compared with the shape of the cutter 51 shown in FIG. 11.

This roundishness keeps the cutter difficult to wear during machining, and also from this point the manufacturing error of the male rotor Mis difficult to occur.

As set forth hereinabove, the screw rotor according to the present invention, which comprises a female rotor and a male rotor formed engage ably with the female rotor, includes a leading-side tooth flank of the female rotor, the leading-side tooth flank being formed by a curved line which is generated by an arbitrarily determined tooth tip portion of the male rotor; an arcuate tooth tip outer periphery surface of the female rotor, with a sealing edge being formed at a leading-side end portion of the tooth tip outer periphery surface; a leading-side tip tooth flank of the female rotor interposed between the leading-side tooth flank of the female rotor and the tooth tip outer periphery surface of the female rotor, the leading-side tip tooth flank of the female rotor being connected smoothly to the leading-side tooth flank and the tooth tip outer periphery surface of the female rotor and formed at a curvature which becomes larger as the leading-side tip tooth flank approaches the tooth tip outer periphery surface of the female rotor; and a follow-up-side tooth flank of the male rotor generated by a tooth flank which is determined by both the leading-side tip tooth flank and the tooth tip outer periphery surface of the female rotor.

According to this construction it is possible to improve the working efficiency and dimensional accuracy in the rotor machining work and also improve the durability of the rotor machining cutter:

Claims

1. A screw rotor comprising a female rotor and a male rotor formed engageably with the female rotor, said screw rotor including a leading-side tooth flank of the female rotor, said leading-side tooth flank being formed by a curved line which is generated by an arbitrarily determined tooth tip portion of the male rotor; an arcuate tooth tip outer periphery surface of the female rotor, with a sealing edge being formed at a leading-side end portion of said tooth tip outer periphery surface; a leading-side tip tooth flank of the female rotor interposed between said leading-side tooth flank of the female rotor and said tooth tip outer periphery surface of the female rotor, said leading-side tip tooth flank of the female rotor being connected smoothly to the leading-side tooth flank and the tooth tip outer periphery surface of the female rotor and formed at a curvature which becomes larger as the leading-side tip tooth flank approaches the tip outer periphery surface of the female rotor; and a follow-up-side tooth flank of the male rotor generated by a tooth flank which is determined by both said leading-side tip tooth flank and said tooth tip outer periphery surface of the female rotor.

2. A screw rotor according to claim 1, wherein said leading-side tip tooth flank of the female rotor is formed at a curvature which becomes larger continuously as the leading-side tip tooth flank approaches said tooth tip outer periphery surface of the female rotor.

3. A screw rotor according to claim 1, wherein said leading-side tip tooth flank of the female rotor is formed at a curvature which becomes larger discontinuously as the leading-side tip tooth flank approaches said tooth tip outer periphery surface of the female rotor.

4. A screw rotor according to claim 3, wherein said leading-side tip tooth flank of the female rotor is formed at two curvatures which become larger discontinuously as the leading-side tip tooth flank approaches said tooth tip outer periphery surface of the female rotors

5. A screw rotor according to claim 4, wherein, at the portion of the smaller curvature of the two curvatures, the ratio of curvature radius to the radius of a pitch circle is 0.2 or more.

6. A screw rotor according to claim 1, wherein said sealing edge is formed outside a pitch circle of the female rotor.