Screw assembly for a three-screw pump and a three-screw pump including the assembly

Abstract

The triple screw pump (10) comprises a central screw (2) and at least one lateral screw (3), both of which are provided with one or more helical threads, the lateral screw (3) being oriented along the central screw axis (z c ) and the transverse screw axis (z l ) and is arranged to mesh with the central screw (2), and the axis distance between the central screw (2) and the lateral screw (3) is set to the outer diameter (Φ ce ) but less than 3 / 5.

Description

[Technical Field]

[0001] The present invention relates to a screw assembly for positive displacement gear pumps, particularly for three-screw pumps. The present invention also relates to a three-screw pump including the above-mentioned screw assembly.

[0002] This invention can find useful applications in various industrial fields where gear pumps, particularly three-screw pumps, have conventionally been used.

[0003] While lifting systems are a typical application where three-screw pumps are highly valued, they are also widely used in a variety of other fields, including power hydraulics, lubrication, cooling, filtration, and transfer. As a non-limiting example, other industrial sectors where three-screw pumps are applied, in addition to lifting systems, include oil and gas, chemicals, navy, transportation, agrofood, power generation and alternative energy, paper industry, and pharmaceutical industry. [Background technology]

[0004] Designed in 1923 by Swedish engineer Carl Montelius, the three-screw pump is a positive displacement pump now widely used in various industrial sectors. In fact, this pump boasts remarkable overall efficiency, good reliability, a reasonable price, and low levels of acoustic emission and vibration in fluid transmission.

[0005] A three-screw pump has a set of three screws: a central drive screw and two laterally driven screws. Preferably, the screws, each having two helical threads, are mounted in parallel within the casing and mesh with each other to create a sealed volume between the body and the casing. The number of closed chambers thus formed is directly proportional to the length of the screw (also called the rotor) and inversely proportional to the pitch of the helical threads. The closed chambers are occupied by the working fluid that moves continuously from the suction port to the discharge port as the screw rotates.

[0006] The profiles of the three screw sets are designed such that only the driving screw transmits pressure. Due to the configuration of the pump, this screw is not exposed to radial forces, and thus exhibits the excellent overall efficiency mentioned above. As described above, the two driven screws idle and are guided by the pressurized fluid. The only impediments to their rotation are the viscous friction with the working fluid and the sliding friction between the central screw and the casing in which they are accommodated. For this reason, the tooth flanks of the screws experience almost no wear even after long periods of operation.

[0007] Even now, nearly one century after its invention, the three-screw pump still exhibits the characteristic appearance devised by its creator, and is characterized by the typical ratio of the diameters of the surface profiles of the central driving screw and the lateral driven screws. Φ li and Φ le respectively represent the inner diameter and the outer diameter of the lateral screw, Φ ci and Φ ce respectively represent the inner diameter and the outer diameter of the central screw, and the dimensions Φ li :Φ le :Φ ci :Φ ce actually follow the ratio of 1:3:3:5, which is considered optimal because it represents the most optimal possible ratio between the area occupied by the fluid and the area of the solid defined by the screw material.

[0008] With this ratio, it should be noted that the outer diameter Φ of the lateral screw le and the inner diameter Φ of the central screw ci are always strictly equal. This equality of these diameters is regarded as an absolute axiom in the prior art, and all designs of three-screw pumps are based on this.

[0009] The circle identified by the aforementioned diameter is the pitch diameter used to generate the curves that form the ideal profile, that is, the profile before the modifications generally adopted to remove sharp edges represent. The reason for choosing this design is the consideration that two base cylinders of equal diameter, rotating at equal tangential velocities and opposite angular velocities, roll on each other without slipping, resulting in less dissipation of heat and energy.

[0010] Furthermore, the inner diameter of the horizontal screw Φ li reducing the outer diameter Φ of the horizontal screw relative to le does not provide any advantages, because in addition to causing rolling accompanied by sliding wear and reduced efficiency, it also results in a reduction in the empty cross-sectional area where working fluid is trapped, that is, a reduction in pump displacement. Conversely, the inner diameter of the horizontal screw Φ li the increase in the outer diameter Φ of the horizontal screw relative to le leads to interpenetration of the helicoidal profiles generated during rotation of the screws, which seemed impossible in prior art research.

[0011] Therefore, when the above equal diameters are selected, in the prior art, the tooth flanks of the central screw and the horizontal screw are obtained by applying an epitrochoid equation, and the epitrochoid is obtained from a fixed point located at a distance p from the center of a circle with radius r, to a circle with radius r b it is a roulette curve obtained by connecting points described in space by rolling said circle on the outside of another circle of . The epitrochoid defines the cross-sectional shape of the tooth flank of each respective screw, and by advancing along the rotation axis of the screw, the shape rotates continuously to define a helix.

[0012] The known parametric equation of the epitrochoid is as follows. TIFF0007920203000001.tif17150 TIFF0007920203000002.tif9150

[0013] The polar equation is as follows. TIFF0007920203000003.tif9150

[0014] In Figure 5, with respect to the prior art, R1 and d1 are beside The parameters relating to the structure of the screw tooth surface are shown, and R2 and d2 are center This shows the parameters related to the structure of the screw tooth surface. From the considerations mentioned earlier, the base radius r is the same for both structures. As can be seen, the radii R1 and R2 of the rotation circle are equal to r in both structures. Furthermore, in the configuration of the central screw tooth surface, the trace point is located at the end of the rotation circle radius r1, and the resulting curve is called an epicycloid.

[0015] Therefore, the only design parameter to be set is the distance from the center of point d2, which is used to draw the tooth surface of the transverse screw and determine the outer diameter of the central screw and the interior of the transverse screw, respectively. The selection of this parameter aims to optimize the volume, i.e., the volume of the screw occupied by the fluid, without affecting the mechanical strength of the screw.

[0016] Specifically, the common ratio of screw diameters, 1:3:3:5, can be obtained by selecting a value for d2 equal to 5 / 3d1, for example, by selecting the following parameters: R1 = R2 = 1.5 r=1.5 d2 = 2.5 (Generates the tooth surface of the driven screw) d1 = 1.5 (to generate the tooth surface of the lead screw or drive screw)

[0017] Because these profiles are similar, by using a simple scaling effect to create this basic relationship, we can obtain a profile of any size.

[0018] It should be noted that the ideal profile generated by the epitrochoid equation has sharp edges. These edges are easily deformed. If the edges are deformed, there is a risk of noise and abnormal vibrations occurring during pump operation, or irreparable damage to the pump itself. Furthermore, it is difficult to manufacture the edges with tooling precision, and as a result, localized shape errors can cause undesirable difficulties in screw meshing.

[0019] For the reasons stated above, in prior art, the ideal profile is generally modified by chamfering the aforementioned sharp edges, especially those of the driven screw that are sharper and potentially more critical. Chamfering can be done in a simple way by cutting the edges in a straight line, or in a more sophisticated way by using an arc-shaped or elliptical arc-shaped connecting profile. The latter solution minimizes leakage and volume loss.

[0020] Clearly, introducing the above geometric corrections results in the loss of complete conjugation along the screw tooth surface lines, requiring a complete recalculation of the profiles of both the driven and driving screws.

[0021] Prior art documents EP1655491A2, DE102009028004A1, and EP0209984A1 disclose a three-screw pump based on prior art.

[0022] As discussed in this chapter regarding prior art, it should be noted that the three-screw pump originated in the early 1900s, and the screw profile has remained largely unchanged to this day. Improvements introduced to date have always involved structural or material modifications.

[0023] On the other hand, such widely used machines always have a need for improvement, particularly in terms of increasing capacity and reducing radial and axial dimensions.

[0024] Therefore, the technical problem of the present invention is to provide a screw assembly and a corresponding three-screw pump having a significantly larger flow rate than conventional pumps of similar size. [Overview of the Initiative]

[0025] The fundamental solution of this invention is the conventional diameter Φ li :Φ le :Φ ci :Φ ce The goal is to revise the ratio of 1:3:3:5 and provide a screw assembly and a corresponding three-screw pump.

[0026] The applicant has observed in practice that it is possible to deviate, at least substantially, from this ratio, which is considered the best compromise between pump capacity and rotor mechanical resistance in the prior art.

[0027] The prior art ratio of 1:3:3:5 means that the distance s between the axes of the central screw and the transverse screws is equal to the outer diameter Φ of the central screw. ce This defines it as being equal to 3 / 5 of . The distance between the axes is actually determined by the sum of the outer radius of the transverse screw and the inner radius of the central screw. Distance between axes s and outer diameter Φ of the central screw ce When the ratio between them decreases, the same diameter Φ ce Note that this defines a larger, more useful area for capturing the working fluid. Furthermore, reducing the distance s between the axes reduces the radial dimensions of the pump. Ideally, the distance s between the axes should be equal to the outer diameter Φ of the central screw. ce It can be reduced to a value equal to half of that. However, this value is equal to the inner diameter Φ of the lateral screw. li Since it matches zero, it cannot be reached specifically.

[0028] On the other hand, in prior art, for reasons of structural rigidity, a ratio of s / Φ lower than 3 / 5 is used. ce The use of this has always been avoided. In fact, as the ratio decreases, the inner diameter of the lateral screw Φ li In other words, the core that needs to be mechanically robust becomes drastically smaller.

[0029] However, the applicant stated that the inner diameter of the lateral screw Φ li The decrease can be compensated for by appropriately reducing the opening angle of the tooth surface β in the lateral screw. The opening angle is defined as the central angle between two intersections of the epitrochoids that create the shape of the screw's pitch circle spanning the hollow portion into which the working fluid can be filled, in terms of the screw's cross-sectional shape. The opening angle β of the teeth of the lateral screw is uniquely related to the same opening angle α of the teeth of the central screw. The applicant determined that the change in the angle does not change the overall area of ​​capture of the working fluid, i.e., the capacity is invariant with respect to the selection of the angle. Therefore, the opening angle β of the tooth surface can be conveniently selected by, in particular, preferably keeping this angle below 90° in order to allow for sufficient mechanical strength of the screw.

[0030] Thanks to these views, the ratio s / Φ ce This is redefined and preferably falls within 52% to 56%, ideally equal to 54%.

[0031] Therefore, the technical problems revealed above are solved by the screw assembly according to claim 1 and by the respective three-screw pumps according to claim 15.

[0032] Therefore, this technical problem is solved by a screw assembly for a three-screw pump, comprising a central screw and at least one transverse screw, both having one or more helical threads, wherein the transverse screw has an axis parallel to the axis of the central screw and is arranged to mesh with the central screw, and the distance between the axes of the central screw and the transverse screw is greater than half and less than 3 / 5 of the outer diameter of the central screw.

[0033] As described above, the distance between the axes of the central screw and the transverse screws is preferably within 52% to 56% of the outer diameter of the central screw, and more preferably equal to 54%.

[0034] The outer diameter of the central screw is preferably 5 times or more, more preferably 10 times or more, the inner diameter of the transverse screw.

[0035] center The inner diameter of the screw is preferably included in 60% to 99% of the outer diameter of the transverse screw, more preferably in 68% to 98%, and even more preferably in 85% to 92%.

[0036] Preferably, the inner diameter of the transverse screw is smaller than the diameter of each pitch circle, and the outer diameter of the transverse screw is larger than the diameter of each pitch circle.

[0037] Preferably, the outer diameter of the transverse screw is between 1 and 1.3 times the diameter of each pitch circle, more preferably between 1 and 1.2 times, and even more preferably, the outer diameter of the transverse screw is equal to 1.1 times the diameter of each pitch circle.

[0038] The features and advantages of the gear wheel and apparatus of the present invention will become apparent from the following description of its embodiments, which are given as non-limiting examples with reference to the accompanying drawings. [Brief explanation of the drawing]

[0039] [Figure 1] Figure 1 schematically shows a three-screw pump that can characterize the screw assembly according to the present invention. [Figure 2] Figure 2 schematically shows a portion of the central screw of the screw assembly according to the present invention in a side view. [Figure 3] Figure 3 schematically shows a portion of the central screw of the screw assembly according to the present invention in a side view. [Figure 4] Figure 4 shows a cross-section of the screw assembly according to the present invention in its operating configuration, where the fluid capture region is identified by the meshed portion. [Figure 5]Figure 5 shows the generation of the screw profile in a prior art three-screw pump. [Figure 6] Figure 6 shows the first step of a conceptual procedure for generating a tooth surface profile in a screw assembly according to the present invention. [Figure 7] Figure 7 shows the second step of a conceptual procedure for generating a tooth surface profile in a screw assembly according to the present invention. [Figure 8] Figure 8 shows the third step of a conceptual procedure for generating a tooth surface profile in a screw assembly according to the present invention. [Figure 9] Figure 9 shows the fourth step of a conceptual procedure for generating a tooth surface profile in a screw assembly according to the present invention. [Figure 10] Figure 10 compares the profile of the central screw according to the present invention with the profile of the central screw according to the prior art. [Figure 11] Figure 11 compares the profile of the transverse screw according to the present invention with the profile of the transverse screw according to the prior art. [Figure 12] Figure 12 compares the profile of the central screw according to the present invention with the profile of the central screw according to the prior art, with the auxiliary fluid capture region indicated by the shaded area. [Figure 13] Figure 13 compares the profile of the transverse screw according to the present invention with the profile of a transverse screw according to the prior art, with the auxiliary fluid capture region identified by the hatched area. [Figure 14] Figure 14 shows the forces acting on a rotor driven by a typical three-screw pump. [Modes for carrying out the invention]

[0040] Referring to Figure 1 above, the three-screw pump is shown as a whole by reference no. 10, and reference no. 1 shows the screw assemblies 2 and 3 assembled on it. As previously stated, the present invention relates in particular to the screw profiles 20 and 30, 2 and 3, which in Figures 10-13 face the corresponding profiles 20' and 30' of the prior art. The new profiles 20 and 30 define, in cross-section, the capture volume V in which the fluid pumped is captured, relative to the corresponding profiles 20' and 30' of the prior art.

[0041] It should be noted that the figures are schematic diagrams and are not drawn to a fixed scale, but rather to highlight key features of the invention. Furthermore, the figures show elements schematically, as their shapes may vary depending on the desired application. It should also be noted that in the drawings, the same reference numerals refer to elements with the same shape or function.

[0042] In known embodiments, a three-screw pump 10 includes a pump body 5 having a suction port S and a discharge port D. Inside the pump body is a screw assembly 1 having a lead central screw 2 integrated with the drive shaft 4 and two driven lateral screws 3. The axis z of the lateral screws 3 l and the axis z of the central screw 2 c The screws are parallel to each other and mesh with each other. The rotational motion of the central screw 2 thus moves the two transverse screws 3, and as shown in Figure 4, the fluid F is transported from the suction port S to the discharge port D in the space enclosed between the opposing screw threads.

[0043] The central screw 2 has a fixed pitch p c It has two threads 21 and 22, and the lateral screw 3 also has the same pitch p as the central screw 2. l It has two screw threads.

[0044] Therefore, the profile 20 of the central screw 2 has, in cross-section, two circular apex portions connected to a cylindrical base by a convex tooth surface.

[0045] The profile 30 of the lateral screw 3 also has, in cross-section, two circular apex portions connected to a cylindrical base by a significantly recessed tooth surface.

[0046] Note that, by known methods, the two transverse screws 3 are equivalent to each other or have the same profile 30.

[0047] As described above, the present invention relates to specific shapes of the tooth surface profiles 20 and 30 of screws 2 and 3.

[0048] Preferred embodiments described herein illustrate preferred shapes of the profiles and how they are obtained from prior art profiles.

[0049] destination In rowing technology, the inner diameter of the lateral screws corresponds to 1 / 3 of their respective outer diameters, and the outer diameter of the central screw corresponds to 5 / 3 of its inner diameter. Therefore, a typical diameter ratio is 1:3:3:5.

[0050] To obtain a new profile, first modify the above ratios to identify new parameter settings that can increase the pump capacity without compromising the mechanical resistance of the screw. The new ratio between diameters Φ' is shown in Figure 6. li :Φ' le :Φ' ci :Φ' ce The ratios were conveniently selected as 0.4:2.7:2.7:5, which allows for an increase of approximately 7% in the suction section. Following the proposed parameter setting for diameter, the inter-axis distance S was reduced from 3 to 2.7 compared to the prior art.

[0051] Starting with new parameters, the ideal profiles of the two screws are generated using the epitrochoid equation described in the prior art analysis. As previously mentioned, an epitrochoid is a curve obtained by connecting points described in space by rolling a circle outside another circle, starting from a fixed point at a constant distance from the center of the radius circle. In this case, the distance from the circle and the radius of the circle are determined by the inner and outer diameters selected for the two screws. The epitrochoid is circumscribed and inscribed in the circle defined by the inner and outer diameters selected for both screws, determining the ideal profile shown in Figure 7.

[0052] Another parameter to be determined is the starting points p, p', and p'' of the epitrochoid formation. In fact, the parameter characterizing the screw is the angle α determined by the chord connecting the two consecutive starting points p' and p'' of the epitrochoid that generate the profile of the central screw 2. This value is uniquely related to the corresponding angle β of the other screw 3. The angles, defined as the tooth opening angle α and the tooth surface opening angle β, define the length of the arc connecting the tooth surfaces on the outer shape of the central screw 2, and the length of the arc between two consecutive teeth of the transverse screw 3. On the one hand, they determine the cylindrical surface that slides in contact with the screw housing, and on the other hand, they determine the mechanical strength of the helix defined by the screw. The applicant has determined through geometric analysis that the volume effective in capturing the working fluid is invariant with respect to the selection of the tooth opening angle α and the tooth surface opening angle β. Thus, the angles can be arbitrarily selected based on tribological and mechanical considerations without affecting the pump's capacity.

[0053] Next, an additional geometric shape g is applied to the ideal profile of the driven lateral screw 3. The additional geometric shape g shown in Figure 8 extends outward from the pitch diameter and onto the tooth surface f defined by the epitrochoid equation, which is the outer diameter Φ' of the lateral screw set earlier. le A diameter larger than that, i.e., the pitch circle diameter C pl Larger diameter Truncated circle (truncation (circle) Ct The additional geometric shape g defines a face c of the screw profile, which connects to the tooth surface at the previously identified point p. The connection point p between the face c defined by the epitrochoid and the tooth surface f defined by the additional geometric shape is preferably an inflection point rather than an angle point (an angle point being a point of non-differentiability of the first kind). The additional geometric shape g can be appropriately selected according to design choices; for example, it can be an elliptic curve or a spline function.

[0054] Once the final profile of the transverse screw 3 is obtained, the profile of the central screw 2 is determined by interpolation. The two final profiles are shown in Figure 9. As you can see, at the base of the surface c' of the central screw 2, which is defined by the epitrochoid, there is a pitch circle C pc A new connecting tooth surface f' is formed for the inner arc. Therefore, redefining the profile will change the inner and outer diameters of the two screws. In particular, center Screw inner diameter Φ ci The outer diameter of the horizontal screw is Φ le It became smaller. Using the parameter settings mentioned above, the final diameter Φ li :Φ le :Φ ci :Φ ce The ratio becomes 0.4:2.97:2.43:5.

[0055] The modifications made to the ideal profile shown in Figure 7 result in an additional approximately 10% increase in pump capacity for the same screw diameter. Therefore, compared to the prior art, the overall capacity increase is approximately 17%. Furthermore, the reduced distance between the screw axes leads to a reduction in the radial dimensions of the pump.

[0056] The improvements mentioned above can be clearly seen in Figures 12 and 13. In fact, the hatched areas indicate that even with the same outer diameter of the screw, the free frontal volume that the fluid pumped up by the pump can occupy has increased, resulting in an increased capacity.

[0057] The advantages of the pump according to the present invention stem particularly from its compact radial dimensions, but for the same flow rate, the axial dimensions also become more compact due to the shorter screw pitch.

[0058] Furthermore, because the pump's structure requires less material, it has the advantage of lower manufacturing costs.

[0059] Other advantages of the pump according to the present invention relate to its performance. In particular, while the volumetric efficiency of the pump remains the same, the pressure ripple is improved, noise is reduced, and the net suction head (NPSH) is lower.

[0060] Clearly, those skilled in the art may make several modifications and alterations to the gears and apparatus described above to meet incidental and specific needs, all of which fall within the scope of protection of the present invention as defined below.

Claims

1. It includes a central screw (2) having one or more helical threads (21, 22) and two transverse screws (3), Each of the lateral screws (3) is connected to the central screw axis (z ｃ ) the horizontal screw axis (z) parallel to the ｌ ) has and is arranged to mesh with the central screw (2), The aforementioned horizontal screw shaft (z ｌ ) and the central screw shaft (z ｃ The distance between the two is the outer diameter (Φ) of the central screw (2). ｃｅ ) is greater than half and less than 3 / 5, The cross-sectional profile of the transverse screw (3) has a tooth surface (f) along the epitrochoid, and the tooth surface (f) has a truncated circle (C) with a diameter larger than the diameter of the pitch circle (Cp1) of the transverse screw (3) via a surface (c) defined by the epitrochoid. ｔ ) is connected, The connection point between the tooth surface (f) and the surface (c) is the inflection point of the cross-sectional profile of the transverse screw (3). Screw assembly (1) of a three-screw pump (10).

2. The lateral screw shaft (z ｌ ) and the central screw shaft (z ｃ ) is included in 52% to 56% of the outer diameter (Φ ｃｅ ) of the central screw (2) The screw assembly (1) according to claim 1.

3. The aforementioned horizontal screw shaft (z ｌ ) and the central screw shaft (z ｃ The distance between the two is the outer diameter (Φ) of the central screw (2). ｃｅ ) is 54% The screw assembly (1) according to claim 2.

4. The outer diameter (Φ) of the central screw (2) ｃｅ ) is the inner diameter (Φ) of the horizontal screw (3) ｌｉ ) is more than 5 times A screw assembly (1) according to any one of claims 1 to 3.

5. The outer diameter (Φ) of the central screw (2) ｃｅ ) is the outer diameter (Φ) of the horizontal screw (3). ｌｅ ) is larger than the inner diameter (Φ) of the central screw (2) ｃｉ ) is the outer diameter (Φ) of the horizontal screw (3). ｌｅ Smaller than ) A screw assembly (1) according to any one of claims 1 to 4.

6. The inner diameter (Φ) of the central screw (2) ｃｉ ) is the pitch circle (C) of the central screw (2). ｐｃ The diameter is smaller than the outer diameter (Φ) of the horizontal screw (3). ｌｅ ) is the pitch circle (C) of the transverse screw (3). ｐｌ Larger than the diameter of ) A screw assembly (1) according to any one of claims 1 to 5.

7. The outer diameter (Φ) of the aforementioned horizontal screw (3) ｌｅ ) is the pitch circle (C) of the transverse screw (3). ｐｌ ) is included in a range of 1 to 1.3 times the diameter of the aforementioned. The screw assembly (1) according to claim 6.

8. The surface (c) of the transverse screw (3) is curved and does not pass through an angle point which is a non-differential point of the first kind, and the tooth surface (f) and the truncation circle (C ｔ ) Connected A screw assembly (1) according to any one of claims 1 to 7.

9. The two horizontal screws (3) are equal to each other, and each is on the central screw axis (z ｃ The horizontal screw axis (z) parallel to ) ｌ ) has and is configured to mesh on both sides of the central screw (2) A screw assembly (1) according to any one of claims 1 to 8.

10. The central screw (2) has an equal pitch (p ｃ It includes a first thread (21) and a second thread (22) having ), The two transverse screws (3) have equal pitch (p ｌ It includes a first thread (31) and a second thread (32) having ), The pitch (p) of the threads of the central screw (2) ｃ ) is the pitch (p) of the threads of the horizontal screw (3). ｌ ) is the same as The screw assembly (1) according to claim 9.

11. Pump body (5) and Suction port (S), Outlet (D), The screw assembly (1) according to claim 9 or 10, The central screw (2) and the lateral screw (3) are rotatably arranged within the pump body (5) and are meshed together. The rotation of the central screw (2) and the lateral screw (3) moves the fluid (F) from the suction port (S) to the discharge port (D). Three-screw pump (10).

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