Balancing of axial forces in screw pumps with drive screw and idler screws

Abstract

The present invention discloses a screw pump comprising a pump casing and a pump screw set disposed therein. The pump screw set consists of one or more drive screw(s) and at least one idler screw. The idler screw(s) rotating in the counter direction of the drive screw(s) during the operation of the screw pump comprise radially protruding flanged portions which lie with their inlet side bearing surfaces on an outlet side bearing surface of a radially protruding flanged portion provided on the main shaft of a drive screw to balance the axial forces acting on and pushing down the idler screws towards the low pressure fluid supply zone. One or more orbital groove is provided on one of the bearing surfaces at a radial distance from the radially outer edge of the bearing surface.

Description

[0001] BALANCING OF AXIAL FORCES IN SCREW PUMPS WITH DRIVE SCREW AND IDLER SCREWS

[0002] Technical Field of the Present Invention

[0003] The present invention relates to balancing of axial forces in screw pumps with a drive screw and idler screws.

[0004] Background of the Present Invention

[0005] Screw pumps with idler screws are generally equipped with three screws consisting of one rotationally driven 'drive screw' and two 'idler screws' that take the movement from the drive screw. A special profiling of the screw flanks allows the three screws to form sealed chambers in the pump. Throughout this description, the exemplary screw pump with one drive screw and two idler screws will be discussed.

[0006] As the screws rotate, the fluid entering through the pump inlet is axially and continuously displaced from the supply side to the discharge side of the pump causing the build-up of extreme high pressures at the high pressure discharge zone at the outlet side of the screw pump and the screw flanks, while the low pressure fluid supply zone at the inlet side of the screw pump remains under low pressure. The axial forces generated during the operation of the pump due to the pressure difference between the high pressure discharge zone at the outlet side of the screw pump and the low pressure fluid supply zone at the inlet side of the screw pump, and acting on both the drive screw and idler screws will push the screw shafts towards the low pressure fluid supply zone of the screw pump.

[0007] The drive screw is supported and axially aligned in the pump casing by at least one bearing located on the top of the screw between the high pressure discharge zone at the outlet side of the screw pump and the sealed area with the atmospheric ambient pressure, enabling it to rotate around a longitudinal rotation axis. The axial forces acting on the drive screw and pushing down the screw towards the low pressure fluid supply zone are substantially balanced by axial forces in opposite direction acting on a labyrinth surface at a level of a labyrinth structure of the drive screw and pushing the drive screw shaft in the direction of the sealed area with the atmospheric ambient pressure, wherein other solutions for balancing the axial forces acting on the drive screw are not excluded. For the same reason, also the axial forces acting on the idler screws and pushing down the idler screws towards the low pressure fluid supply zone are tried to be balanced axially by using different solutions, in order to prevent damage to the screws and the pump. In existing applications known in the art, there are different solutions for balancing the axial forces acting on and pushing down the idler screws towards the low pressure fluid supply zone.

[0008] In applications where high pressure is required, the most commonly used solution is to support the idler screws by leaning the input side ends of the idler screw shafts against discs provided on the base of the pump casing.

[0009] During the operation of the screw pump, the rotation of the idler screws causes in combination with the axial forces acting on and pushing down the idler screws towards the low pressure fluid supply zone, high friction forces between the respective input side bearing ends of the idler screw shafts and the bearing discs provided on the base of the pump casing, limiting the use of this solution without any problems especially to pumping pressures up to a range of 40 bar.

[0010] Another solution is to align the idler screws with the drive screw by a suspension system. In this solution, the idler screws are suspended at their outlet side shaft region, by means of respective bearing surfaces of circumferential flanged portions protruding radially from their main shaft and lying on the bearing surface of a circumferential flanged portion of the drive screw provided at an appropriate level of the drive screw shaft.

[0011] During the operation of the screw pump, both of the idler screws rotate in a direction opposite to the rotation direction of the drive screw, causing in combination with the axial forces acting on and pushing down the idler screws towards the low pressure fluid supply zone, the generating of high friction forces between the respective bearing surfaces at the flanged portions of the idler screws and the bearing surface at the flanged portion of the drive screw, also limiting the use of this solution without any problems especially to pumping pressures up to a range of 40 bar. At higher pressures, the axial forces acting on and pushing down the idler screws will increase and the friction forces on the bearing surfaces will cause wear.

[0012] Another wear factor at higher pressures is the material hardness of the screws used. The material hardness required to prevent the bearing surfaces from being corroded by the forces acting at high pressures around 100 bar should be between 60 HRC and 65 HRC.

[0013] Nowadays, screw pumps with much higher pumping capacity are used for pumping systems for much higher pressures up to 100 bar. Since the forces acting on and pushing down the screws will increase when the pump pressure increases to higher values, there is a need to improve the techniques used to achieve the balancing function in the screw pumps for much higher pressures up to 100 bar. To achieve this balancing function in the screw pump with the fixed bearing discs provided on the base of the pump casing, an additional channel connection is established from the high pressure zone (pump outlet side) to the low pressure zone (pump inlet side) from inside or outside the pump casing. With this approach, a pressurized fluid transfer through additional channels is provided, reaching from the high pressure zone (pump outlet) to the low pressure zone (pump inlet) and a film layer is formed between the rotating inlet side end bearing surface of the respective idler screw shaft and the fixed bearing disk surface, thus enabling hydrostatic bearing. This solution is complex, elaborate and costly and requires much more mechanical operation compared to a suspension system.

[0014] The Patent WO 2017 / 189.022 A1 with the title "Modular Thrust-Compensating Rotor Assembly" is the document defining the state of the art for achieving the balancing function in the suspension system and forms the preamble of claim 1.

[0015] This document proposes to provide "a tapered bearing surface configured to define a wedge- shaped, radial gap axially intermediate the power rotor and the idler rotor” for allowing lubricative fluid supply on bearing surfaces between the drive screw and the idler screw to create a hydrodynamic bearing effect. This solution contributes to the balancing of the axial forces to a certain degree but causes further problems due to the very small direct contact area between the drive screw and the idler screw on a slender circular line. Namely, the necessarily sharp or small edge shaped contacting portion of one of the rotors is vulnerable for deterioration or breakdown because the sharp or small edge with very small direct contact surface has to withstand much higher axial forces under comparable pressures.

[0016] Objects of the Present Invention

[0017] Under the light of the above-mentioned explanations, the aim of the present invention is to improve balancing of axial forces in screw pumps with a drive screw and at least one idler screw with respect to the state of the art, through providing a large surface lubricative fluid film on the bearing surfaces between the circumferential flanged portion of an idler screw and the circumferential flanged portion of the drive screw.

[0018] In line with this aim, an object of the present invention is to provide on the bearing surface of the circumferential flanged portion of the respective idler screw or the bearing surface of the circumferential flanged portion of the drive screw an orbital groove at a radial distance from the radially outer edge, for being able to supply lubricative fluid between the bearing surfaces.

[0019] Summary

[0020] The aim and the object of thew present invention are achieved through a screw pump according to the claim 1 comprising a pump casing, a set of pumping screws within the pumping space consisting of one or more drive screws and one or more idler screws driven by the drive screw through radially intermeshing threaded portions, wherein the idler screws are supported through the inlet side bearing surfaces of the flanged portions of their shaft leaning on the outlet side bearing surface of the flanged portion of the drive screw shaft, characterised in that at least one of said bearing surfaces is provided with an orbital groove at a radial distance from the outer edge of the bearing surface.

[0021] The orbital groove conveys pressurized lubricative fluid of the high pressure environment between relatively moving bearing surfaces of the drive screw and the idler screws, functioning similarly to a segmented hydrostatic bearing. In addition, the large surface area of the bearing surfaces with the lubricative fluid film in between serves to provide hydrodynamic bearing effect.

[0022] Other advantages of the invention are defined in the appended subclaims.

[0023] Brief Description of the Technical Drawings

[0024] Accompanying drawings are given solely for the purpose of exemplifying a screw pump in which high axial forces created during the operation of the screw pump with drive screw and idler screws are effectively balanced, whose advantages over prior art were outlined above and will be explained in brief hereinafter.

[0025] Figure 1A schematically demonstrates a prior art application of a screw set of a screw pump with the drive screw and idler screws aligned with suspension system, Figure 1 B is a partial longitudinal sectional view showing the screw set of Figure 1 as arranged in the respective screw pump,

[0026] Figure 2 schematically demonstrates in a longitudinal sectional view a prior art application showing the inlet side ends of the idler screws leaning against bearing discs provided on the base of the pump casing, Figure 3 schematically demonstrates in a longitudinal sectional view another prior art application showing the supply of pressurized lubricative fluid to the bearing discs at the low pressure zone,

[0027] Figure 4A is a schematic partial view of the screw set of a screw pump according to the invention showing in a partial longitudinal sectional view a circular orbital groove on the outlet side bearing surface of the flanged portion of the drive screw, at a radial distance from its radially outer edge,

[0028] Figure 4B is a partial perspective view emphasizing the orbital circular groove, Figure 5 is a schematic partial view similar to the one of the Figure 4, with the flanged portions of the screws located at the low pressure zone of the screw pump.

[0029] Detailed Description of the Present Invention

[0030] The following numerals are referred to in the detailed description of the present invention:

[0031] 1 Screw pump

[0032] 2 Screw set

[0033] 10 Screw pump casing

[0034] 11 Drive screw

[0035] 12 Main shaft of the drive screw

[0036] 13 Flanged portion of the drive screw

[0037] 14 Idler screws

[0038] 15 Flanged portions of the idler screws

[0039] 16 High pressure zone

[0040] 17 Labyrinth surface

[0041] 18 Circumferential channels

[0042] 19 Low pressure (fluid supply) zone

[0043] 20 Bearing surface (of the drive screw)

[0044] 21 Bearing surfaces (of the idler screws)

[0045] 22 Circular groove, orbital groove

[0046] 23 Bearing discs

[0047] 24 Lubricative fluid channel

[0048] 25 Through openings The screw pump (1) generally indicated with reference numeral 1 in the Figures is generally used for pumping lubricative fluids like every kind of oils or resins etc.. The screw pump (1) comprises a pump casing (10) and a pump screw set (2) disposed within the pump casing (10). In the exemplary embodiments of the invention shown in the Figures, the pump screw set (2) consists of one drive screw (11) and two idler screws (14), however one skilled in the art would understand that the teaching of the invention is functional with more than one drive screws and / or with one or any other number of idler screws (14).

[0049] As explained in the background description, the axial forces generated during the operation of the pump due to the pressure difference between the high pressure discharge zone and the low pressure fluid supply zone and pushing down the screw shafts towards the low pressure fluid supply zone at the inlet side of the screw pump (1) by acting on both the drive screw (11) and idler screws (14) are balanced in different manner according to the generated pressure level. Figure 1A schematically demonstrates a prior art application of a screw set (2) of a screw pump (1) with one drive screw (11) and two idler screws (14) aligned with suspension system. Here, the two idler screws (14) rotating in the counter direction of the drive screw (11) lie with their radially protruding flanged portion at the outlet side end of the screw flanks, on the radially protruding flanged portion (13) of the main shaft of the drive screw (12) (Figure 1 B).

[0050] During the operation of the screw pump (1), the high pressure generated in the high pressure discharge zone pushes down the screws towards the low pressure fluid supply zone at the inlet of the screw pump (1). For the drive screw (11), the axial forces acting on the drive screw (11) and pushing down the screw towards the low pressure fluid supply zone are balanced by axial forces in opposite direction acting on a labyrinth surface (17) with circumferential channels (18) at a level of a labyrinth structure of the drive screw (11) and pushing up the main shaft of the drive screw (12) towards the sealed area with the atmospheric ambient pressure.

[0051] For the two idler screws (14), the outlet side bearing surface (20) of the radially protruding flanged portion of the drive screw (13) in contact with inlet side bearing surfaces (21) of the radially protruding flanged portions of the idler screws (15) has to countervail the axial forces acting on the two idler screws (14) and pushing down the screws towards the low pressure fluid supply zone. Furthermore, the rotation of the idler screws (14) in a direction opposite to the rotation direction of the drive screw (11) comes in play, which, in combination with the pushing pressure cause high pression and friction forces.

[0052] For a screw pump (1) with suspension system for pumping lubricative fluids, whose pumping screws work in lubricative fluid environment, the contacting flat bearing surfaces (20, 21) of the flanged portions can absorb these forces without having to take special measures for pressures up to 40 bar (Figure 1A, Figure 1 B).

[0053] Figure 2 schematically demonstrates in a longitudinal sectional view another prior art application by which the down ends of the idler screws (14) lean against bearing discs (23) provided on the base of the pump casing (10) instead of being suspended on radially protruding flanged portion of the drive screw (13). This application also is functional for pressures up to 40 bar.

[0054] For higher pressures up to 100 bar, the screw pump (1) of Figure 2 is improved in that an additional lubricative fluid channel (24) connection is established from the high pressure zone (16) (pump outlet) to the low pressure zone (19) (pump inlet) from inside or outside the pump casing (10). By this means, a transfer of high pressure lubricative fluid takes place from the high pressure discharge zone to the bottom portion of the pump, wherein the high pressure lubricative fluid passes through openings (25) provided at the base of the pump and reaches the inlet side ends of the idler screw shafts for balancing the axial forces acting on the idler screws (14) (Figure 3).

[0055] Figure 4A is a schematic partial view of a screw pump (1) according to the invention showing in a longitudinal sectional view an orbital circular groove (22) on the outlet side bearing surface (20) of the flanged portion of the drive screw (13) arranged at a radial distance from the outer edge of the outlet side bearing surface (20). Figure 4B is partial perspective view emphasizing the orbital circular groove (22). As seen, the width of the circular groove (22) in the radial direction is very small in relation to the radial width of the outlet side bearing surface (20). In this most preferred embodiment of the invention, the orbital groove (22) conveys a sufficient amount of pressurized lubricative fluid of the high pressure environment between relatively moving bearing surfaces (20, 21) of the drive screw (11) and the idler screws (14), functioning similarly to a segmented hydrostatic bearing. In addition, a large surface lubricative fluid film is provided on the bearing surfaces (21) of the circumferential flanged portions of the idler screws (15) and the circumferential flanged portion of the drive screw (13), wherein the large surface area of the bearing surfaces (20, 21) with the lubricative fluid film in between serves to provide hydrodynamic bearing effect. Due to the presence of a large surface lubricative fluid film between the bearing surfaces (21) of the circumferential flanged portions of the idler screws (15) and the circumferential flanged portion of the drive screw (13), the direct contact between the bearing surfaces (20, 21) is prevented, and the axial forces acting on the idler screws (14) are effectively balanced. Without points of direct contact, the most of the disturbing friction forces is also eliminated.

[0056] In a preferred embodiment of the invention the bearing surfaces (21) of the circumferential flanged portions of the idler screws (15) and the circumferential flanged portion of the drive screw (13) are perpendicular to the rotation axis of the drive screw (11). This prevents effectively the development of disturbing forces in various directions due to eventual inclinations.

[0057] In a preferred embodiment of the invention the orbital groove (22) at a radial distance from the radially outer edge has a circular shape in its plan view. The circular shape is easy machinable and provides regular and continuous lubricative fluid supply between the bearing surfaces (20, 21). In other embodiments, the orbital groove (22) can present other shapes in its plan view like a serpentine shape etc. for eventual special distribution reasons.

[0058] Figure 5 is a schematic partial view, showing the flanged portion (13) of the main shaft of the drive screw (12) located at the low pressure zone (19) of the screw pump (1). In this case the idler screws (14) don’t need to have flanged portions since the bottom side end surfaces lying on the flanged portion (13) of the mam shaft of the drive screw (12) fulfil the bearing function. In some screw pump (1) designs the convenient mounting space for the flanged portion can be available at the low pressure zone (19) of the screw pump (1). Providing a circular orbital groove (22) on the bearing surface (20) of the flanged portion (13) provides comparable balancing function.

[0059] In an embodiment of the invention the orbital groove (22) is arranged on the bearing surface (20) at equal radial distance to the outer edge and the inner perimeter of the bearing surface (20). This ensures optimal distribution of the lubricative fluid overall on the surface. In other embodiments the orbital groove (22) can be arranged on the bearing surface (20) at different radial distances to the outer edge and the inner perimeter of the bearing surface (20).

[0060] In an embodiment of the invention the circular groove (22) on the bearing surface (20) is V- shaped in its cross-sectional view. In other embodiments the circular groove (22) can present other cross-sectional shapes like U-shape, half circular shape etc.

[0061] In an embodiment of the invention the width of the circular groove (22) in the radial direction is ten times smaller than the radial width of the outlet side bearing surface (20). Thus, a sufficient amount of lubricative fluid is continuously supplied on bearing surfaces (20, 21), while a large surface area remains on bearing surfaces (20, 21) for forming the lubricative fluid film. In other embodiments of the invention, the ratio of the radial width of the circular groove (22) to the radial width of the outlet side bearing surface (20) can be between 1 / 1 to 1 / 30.

[0062] In an embodiment of the invention two circular grooves are provided on the outlet side bearing surface (20) of the flanged portion of the drive screw (13). Thus, better lubricative fluid distribution can be achieved on the bearing surfaces (20, 21). The number of the circular grooves is not limited, however it’s important to bear in mind that larger surfaces of lubricative film on a bearing surface fulfil better the function of balancing.

[0063] In an embodiment of the invention a circular orbital groove (22) is provided on each of the lower bearing surfaces (21) of the flanged portions of the idler screws (15), at a radial distance from their radially outer edges. With this structure the function of balancing is achieved on the same way and is preferred for designs where the machining of the idler screws (14) is easier than the drive screw (11).

[0064] The most important advantage of the invention is that it can provide lubrication only with an orbital groove (22) of small cross-section provided on a bearing surface (20) without requiring difficult mechanical processing such as a through hole on the additional part or casing. In order to prevent wear and damage to the pump, a lubricative fluid film is formed on the bearing surface (20) of the flanged portion of the drive screw (13) and the flanged portions of the idler screws (15).

Claims

CLAIMS1) Screw pump (1) comprising a pump casing (10) and a pump screw set (2) disposed within the pump casing (10), the pump screw set (2) consisting of one or more drive screw and at least one idler screw, wherein the idler screw(s) (14) rotating in the counter direction of the drive screw(s) (11) during the operation of the screw pump (1) comprise radially protruding flanged portions (15) which lie with their inlet side bearing surfaces(21) on an outlet side bearing surface (20) of a radially protruding flanged portion (13) provided on the main shaft of a drive screw (12), in order to balance the axial forces acting on and pushing down the idler screws (14) towards the low pressure fluid supply zone (19), characterized in that an (one or more) orbital groove (22) is provided on one of the bearing surfaces (20, 21) at a radial distance from the radially outer edge of the bearing surface (20, 21).2) Screw pump (1) according to claim 1 , characterized in that the bearing surfaces (21) of the circumferential flanged portions of the idler screws (15) and the circumferential flanged portion of the drive screw (13) are perpendicular to the rotation axis of the drive screw (11).3) Screw pump (1) according to claim 1 , characterized in that the orbital groove (22) at a radial distance from the radially outer edge has a circular shape in its plan view.4) Screw pump (1) according to claim 1 , characterized in that the orbital groove (22) at a radial distance from the radially outer edge has a non-circular shape in its plan view.5) Screw pump (1) according to claim 1 , characterized in that the pump screw set (2) consists of one drive screw (11) and two idler screws (14).6) Screw pump (1) according to claim 1 , characterized in that only one orbital groove (22) is provided on the bearing surface (20) of the radially protruding flanged portion (13) of the main shaft of the drive screw (12).7) Screw pump (1) according to claim 1 , characterized in that a plurality of orbital grooves(22) are provided on the bearing surface (20) of the radially protruding flanged portion (13) of the main shaft of the drive screw (12).8) Screw pump (1) according to claim 1 , characterized in that the circumferential flanged portions of the idler screws (15) and the circumferential flanged portion of the drive screw (13) are provided at the high pressure fluid discharge zone (16) of the screw pump (1).9) Screw pump (1) according to claim 1 , characterized in that the circumferential flanged portions of the idler screws (15) and the circumferential flanged portion of the drive screw (13) are provided at the low pressure fluid supply zone (19) of the screw pump (1).10) Screw pump (1) according to claim 1 , characterized in that the circular groove (22) on the bearing surface (20, 21) is V-shaped in its cross-sectional view.11) Screw pump (1) according to claim 1 , characterized in that the ratio of the radial width of the circular groove (22) to the radial width of the outlet side bearing surface (20, 21) is between 1 / 1 to 1 / 30.

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