SCREW ASSEMBLY FOR A TRIPLE SCREW PUMP AND TRIPLE SCREW PUMP COMPRISING SAID ASSEMBLY

Abstract

Screw assembly (1) for a triple screw pump (10), comprising: a central screw (2) and at least one lateral screw (3), both provided with one or more helicoidal threads, said lateral screw (3) being arranged to mesh with said central screw (2) with a lateral screw axis (z.sub.1) parallel to the central screw axis (z.sub.c), wherein the distance between the axes of the central screw (2) and of the lateral screw (3) is greater than the half and lower than ? of a central screw (2) external diameter (?.sub.ce).

Claims

1. A screw assembly for a triple screw pump, comprising: a central screw and two lateral screws, all provided with one or more helicoidal threads, each lateral screw being arranged to mesh with said central screw with a lateral screw axis parallel to a central screw axis, wherein the distance between the central screw axis and the lateral screw axis parallel is greater than ? and lower than ? of a central screw external diameter.

2. The screw assembly according to claim 1, wherein the distance between the central screw axis and the lateral screw axis is comprised between 52% and 56% of the central screw external diameter.

3. The screw assembly according to claim 2, wherein the distance between the central screw axis and the lateral screw axis is substantially equal to 54% of the central screw external diameter.

4. The screw assembly according to claim 1, wherein the central screw external diameter is greater than 5 times a lateral screws internal diameter.

5. The screw assembly according to claim 1, wherein the central screw external diameter is greater than a lateral screws external diameter, a central screw internal diameter being lower than the lateral screws external diameter.

6. The screw assembly according to claim 5, wherein the central screw internal diameter is comprised between 60% and 99% of the lateral screws external diameter.

7. The screw assembly according to claim 6, wherein the central screw internal diameter is comprised between 85% and 92% of the lateral screws external diameter.

8. The screw assembly according to claim 1, wherein the central screw internal diameter is smaller than a diameter of a respective pitch circle and a lateral screws external diameter is greater than a diameter of a respective pitch circle.

9. The screw assembly according to claim 8, wherein a lateral screws external diameter is comprised between 1 time and 1.3 times a diameter of a respective pitch circle.

10. The screw assembly according to claim 1, wherein a cross-sectional profile of the lateral screws has a flank which follows an epitrochoid, said flank being joined to a truncation circle by a face.

11. The screw assembly according to claim 10, wherein flank and face connect in an inflexion point on the cross-sectional profile of the lateral screw.

12. The screw assembly according to claim 10, wherein said face of the lateral screws is curvilinear and joined to the flank and the truncation circle without angular points (non-differentiable point of the first kind).

13. The screw assembly according to claim 1, wherein the lateral screws are equal to each other and configured to mesh at two sides of the central screw with the lateral screw axis parallel to the central screw axis.

14. The screw assembly according to claim 13, wherein the central screw comprises a first thread and a second thread having an equal pitch, and both lateral screws comprise a first thread and a second thread having an equal pitch, the pitch of the central screw threads being equal to the pitch of the lateral screw threads.

15. A triple screw pump comprising a pump body, a suction port, a discharge port and a screw assembly according to claim 13, wherein the main screw and the lateral screws are arranged in a rotating manner and intermeshed within the pump body, a rotation of said main and lateral screws moving a fluid from the suction port to the discharge port.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] In these drawings:

[0044] FIG. 1 schematically shows a triple screw pump which can feature the screw assembly according to the invention;

[0045] FIG. 2 schematically shows, in a side view, a portion of the central screw of the screw assembly according to the invention;

[0046] FIG. 3 schematically shows, in a side view, a portion of the central screw of the screw assembly according to the invention;

[0047] FIG. 4 shows a cross section of the screw assembly according to the present invention in an operational configuration, with the fluid trapping areas identified by meshed portions;

[0048] FIG. 5 shows a diagram relating to the generation of screw profiles in a triple screw pump of the prior art;

[0049] FIG. 6 shows a first step of a conceptual procedure for generating the flank profile in a screw assembly according to the present invention;

[0050] FIG. 7 shows a second step of a conceptual procedure for generating the flank profile in a screw assembly according to the present invention;

[0051] FIG. 8 shows a third step of a conceptual procedure for generating the flank profile in a screw assembly according to the present invention;

[0052] FIG. 9 shows a fourth step of a conceptual procedure for generating the flank profile in a screw assembly according to the present invention;

[0053] FIG. 10 compares the profile of a central screw according to the present invention with the profile of a central screw according to the prior art;

[0054] FIG. 11 compares the profile of a lateral screw according to the present invention with the profile of a lateral screw according to the prior art;

[0055] FIG. 12 compares the profile of a central screw according to the present invention with the profile of a central screw according to the prior art, with the supplementary fluid trapping area identified by a hatched portion;

[0056] FIG. 13 compares the profile of a lateral screw according to the present invention with the profile of a lateral screw according to the prior art, with the supplementary fluid trapping area identified by a hatched portion;

[0057] FIG. 14 illustrates the forces acting on the rotors driven in a generic triple screw pump.

DETAILED DESCRIPTION

[0058] With reference to the above FIG. 1, a triple screw pump is globally indicated with reference number 10, whereas reference number 1 indicates the screw assembly 2, 3 assembled thereon. As previously described, the present invention specifically relates to the profiles 20, 30 of said screws 2, 3, which in FIGS. 10-13 faces a corresponding profile 20, 30 of the prior art. The new profiles 20, 30 define, in cross-section, a supplementary volume V in which the fluid to be pumped is trapped with respect to the corresponding profiles 20, 30 of the prior art.

[0059] It is worth noting that the figures represent schematic views and are not drawn to scale, but instead they are drawn so as to enhance the important features of the invention. Furthermore, in the figures, the different elements are shown schematically since their shape may vary according to the desired application. It is also worth noting that in the figures identical reference numbers refer to elements that are identical in shape or function.

[0060] In a known manner, the triple screw pump 10 comprises a pump body 5 with a suction port S and a discharge port D. Within the pump body a screw assembly 1 is assembled with a lead central screw 2, integral with a driving shaft 4, and two driven lateral screws 3. The axes z.sub.l of the lateral screws 3 and the axis z.sub.c of the central screw 2 are parallel to each other and the screws mesh with each other. The rotation movement of the central screw 2 thus moves the two lateral screws 3 and carries a fluid F from the suction port S to the discharge port D in the spaces enclosed between the opposite threads, as illustrated in FIG. 4.

[0061] The central screw 2 has two threads 21, 22 with fixed pitch p.sub.c; the lateral screws 3 also have two threads with pitch p.sub.l equal to that of the central screw 2.

[0062] The profile 20 of the central screw 2 thus has in cross-section two circular crest portions, joined to the cylindrical bottom by noticeably convex flanks.

[0063] The profile 30 of the lateral screw 3 also has in cross section two circular crest portions, joined to the cylindrical bottom by noticeably concave flanks.

[0064] It is noticed that, in a known manner, the two lateral screws 3 are equal to each other or have the same profile 30.

[0065] As above mentioned, the present invention relates to the specific shape of the profiles 20, 30 of the flanks of the screws 2, 3.

[0066] The preferred embodiment herein described shows a preferred shape of said profiles, showing how this is obtained from the prior art profiles.

[0067] As described in the corresponding paragraph of the present disclosure, the prior art profiles are made from an equivalence condition between lateral screw internal diameter and lateral screw external diameter. Thus, as shown in FIG. 5, there is a distance between the axes s equal to the lateral screw 2 internal diameter, i.e. to the lateral screw 3 external diameter. Moreover, in the prior art the lateral screw internal diameter is equal to ? of the respective external diameter, and the central screw external diameter is equal to 5/3 of the internal diameter. Therefore, there is the typical ratio between diameters 1:3:3:5.

[0068] To obtain the new profiles, first of all the above mentioned ratio is modified, identifying a new parameterization that allows increasing the capacity of the pump without compromising the mechanical resistance of the screws. This new ratio between diameters ?.sub.li: ?.sub.le: ?.sub.ci: ?.sub.ce, illustrated in FIG. 6, is conveniently chosen as 0.4:2.7:2.7:5, and allows increasing the suction section by about 7%. Following the proposed parameterizations with respect to the diameters, with respect to the prior art the new distance between the axes S has thus been reduced from the value of 3 to the value of 2.7.

[0069] Starting from the new parameters, ideal profiles for the two screws are generated by using the epitrochoid equations as described in the prior art analysis. As previously described, the epitrochoid is the curve obtained by joining the points described in space from a fixed point at a certain distance from the center of a radius circle by rolling said circle outside another circle: the distance from the circle and the radius of the circles are determined in this case by the internal and external diameters chosen for the two screws. The epitrochoid is externally and internally joined to the circles defined by the internal and external diameters chosen for both screws, thus determining the ideal profiles visible in FIG. 7.

[0070] The other parameter to be determined is the starting point p, p, p for the epitrochoid generation. Actually, the parameter that characterizes the screws is the angle ? subtended by the chord that joins two successive starting points p, p of the epitrochoids that generate the profile on the central screw 2: said value is uniquely linked to the corresponding angle ? on the other screw 3. Said angles, hereinafter defined tooth opening angle ? and flank opening angle ?, define the length of the arc of circle that joins the flanks on the external profile of the central screw 2 and the length of the arc of circle between two successive teeth of the lateral screw 3. On the one hand they determine the cylindrical surface in sliding contact with the housing of the screws, on the other hand the mechanical strength of the helix defined on the screw. The applicant has determined, through geometric analyzes, that the useful volume for trapping the working fluid is an invariant with respect to the choice of the opening angles of the tooth ? and of the flank ?. For this reason, the angles may be selected at will just based on tribological and mechanical considerations, without impacting the capacity of the pump.

[0071] Then the application of an additional geometry g above the ideal profile of the driven lateral screw 3 is carried out. Said additional geometry g, represented in FIG. 8, develops outside the pitch diameter, and joins the flank f defined by the epitrochoid equation to a truncation circle C.sub.t, of diameter greater than the lateral screw external diameter ?.sub.le previously set, i.e. of a diameter greater than the pitch circle diameter C.sub.pl. The additional geometry g thus defines a face c of the screw profile, which joins to the flank in the previously identified point p. The connection point p between the face c defined by the epitrochoid and the flank f defined by the additional geometry will preferably be an inflexion point, not an angular point (wherein by angular point a non-differentiable point of the first kind is intended). The additional geometry g can be suitably selected according to the design choices, for example it can be an elliptical curve or a spline function.

[0072] Once obtained the final profile of the lateral screw 3, the profile of the central screw 2 is obtained by interpolation. The two final profiles are illustrated in FIG. 9. As it may be noticed, at the base of face c of the central screw 2 defined by the epitrochoid a connection flank f to a new internal arc with respect to the pitch circle C.sub.pc develops. The redefinition of the profiles thus leads to a variation of the internal and external diameters of the two screws. In particular, the lateral screw internal diameter ?.sub.ci is now smaller than the lateral screw external diameter ?.sub.le. The ratio between the final diameters ?.sub.li: ?.sub.le: ?.sub.ci: ?.sub.ce, by using the previous parametrization, is now 0.4:2.97:2.43:5.

[0073] The modification made with respect to the ideal profiles illustrated in FIG. 7 leads to a further increase in the pump capacity for the same diameter of the screws equal to about 10%. The increase in the overall capacity compared to the prior art is therefore equal to about 17%. In addition, the radial dimensions of the pump decrease due to the reduction in the distance between the axes of the screws.

[0074] The above described improvement can be clearly seen in FIGS. 12, 13; in fact, the hatched area represents an increase in the free frontal volume that can be occupied by the pumped fluid with a consequent capacity increase with the same external diameters of the screws.

[0075] An advantage of the pump according to the present invention results from the particularly compact dimensions, in particular in the radial direction, but also in the axial direction since with the same flow rate the pitch of the screws will be shorter.

[0076] A further advantage comes from the lower amount of material required for the construction of the pump, which results in limited production costs.

[0077] Other advantages of the pump according to the present invention relate to its performance features. In particular, the pump has the same volumetric efficiency but a better pressure ripple, a reduced noise, and a lower net positive suction head (NPSH).

[0078] Obviously, a skilled person can make several changes and variants to the above described gear wheel and the apparatus, in order to meet contingent and specific needs, all of them by the way contained in the scope of protection of the invention as defined by the following.