SCREW PUMP

Description

[0001] The invention relates to a screw pump, comprising a housing and a drive spindle accommodated therein and at least one running spindle meshing with the drive spindle, each of which has two end faces. Screw pumps are used to convey a wide variety of substances, primarily fluid media. A screw pump is known to comprise a housing in which the spindle assembly comprises a drive spindle that extends out of the housing and is coupled to a drive motor, optionally with the interposition of a gear, as well as one or more running spindles, whose spindle profiles mesh with the spindle profile of the drive spindle and which are driven via the drive spindle. The housing in which the spindle assembly is accommodated can be the pump housing, which also closes off to the outside, or a housing designed as an insert in an external housing.

[0002] Typically, one or more spindles, usually two parallel and offset by 180° next to the drive spindle, are axially hydraulically supported. For this purpose, a nozzle orifice can be provided adjacent to the end face of the respective spindle, through which a portion of the fluid to be pumped flows against the spindle end face to build up an axial support pressure, which axially supports the respective spindle. This requires a corresponding housing design, in which a corresponding fluid supply must be provided via suitable channels, as well as the corresponding nozzle orifices, which must also be geometrically designed and configured to generate the appropriate fluid pressure.

[0003] The documents US 2 693 762 A, DE 10 2006 049663 A1, US 2011 / 014079 A1 and DE 78 02 765 U1 disclose screw pumps with measures for compensating and / or absorbing axial forces.

[0004] Screw pumps are increasingly being used in the food and pharmaceutical industries, meaning they are used to pump corresponding fluids such as food or pharmaceutical substances. Working with such substances requires the highest level of hygiene, which is why the screw pumps used must be cleaned at correspondingly short intervals. Due to the complex design of the screw pump with regard to the fluid flow for axial support of the impeller spindles, which includes corresponding channels, etc., it is necessary to remove and disassemble the screw pump and clean it to ensure that all areas are cleaned. This is because the integration of the additional channels, nozzle orifices, etc. results in a considerable volume that does not participate in the actual pumping process, i.e. exists as a kind of dead space, but is nevertheless pressurized with the fluid.

[0005] The invention is therefore based on the problem of providing an improved screw spindle pump.

[0006] To solve this problem, the features of independent claim 1 are provided according to the invention in a screw spindle pump of the type mentioned at the outset.

[0007] In the screw spindle pump according to the invention, no hydraulic axial thrust compensation is provided for the idler spindles. Instead, one or, if two idler spindles are provided, each idler spindle is assigned an axial thrust surface at least on one side, whereby the idler spindle is accommodated with slight axial play in relation to this thrust surface. This thrust surface(s) is therefore located in the actual pump chamber. During operation, the drive spindle is driven. Due to the profile engagement or the hydraulic pressure, one or both idler spindles also rotate with the drive spindle, so that fluid is pumped through the pump chamber. The drive spindle itself is largely hydraulically balanced, which means that no or only a negligible axial force acts on the drive spindle due to operational reasons.This is achieved by ensuring that the pressurised surface of a sealing element that seals the drive spindle to the housing and the pressurised profile surface of the drive spindle profile are essentially the same. Since both surfaces are axially pressurised in different directions, a force equilibrium is established, which means that the drive spindle is hydraulically balanced. During operation, the idler spindles only experience a slight axial offset in the direction of one or more of the contact surfaces due to the pump pressure. If the pump is not reversible, the contact surface is provided on the suction side or the suction-side end of the idler spindle, since the idler spindle shifts slightly towards the suction side during operation. If the pump is reversible, two contact surfaces are provided per idler spindle in order to provide a contact surface on both sides depending on the direction of flow and thus the direction of movement of the idler spindle.If the drive spindle pump is reversible with regard to the delivery direction, this offset occurs either towards one or the other thrust surface, depending on the working direction. The offset is possible due to the low axial play, whereby the axial play can be designed with regard to the resulting maximum offset. The respective end face of the idler spindle can run against the respective axial thrust washer during operation, where it is ideally supported by a thin hydraulic lubricating film or, if it runs against the thrust surface, there is only negligible friction. This means that despite running against the thrust surface, on the one hand there is appropriate support and lubrication via the fluid to be pumped, since the thrust surface is in the pump chamber as described, and on the other hand there is no wear to accommodate.

[0008] The screw pump according to the invention therefore enables appropriate axial support of the impeller spindles, but does not provide any specific measures for this other than the integration of two thrust washers. The only volume through which the fluid to be pumped flows is the pump chamber, which is virtually optimized for dead space. This, in turn, means that the screw pump according to the invention does not need to be disassembled for cleaning, as the cleaning process can be carried out while installed, after which the cleaning fluid can easily flush the pump chamber. This means that so-called "cleaning in place (CIP)" is possible with the screw pump according to the invention.

[0009] As already described, in a non-reversible pump, a thrust surface can be assigned only to the suction-side end face of the impeller spindle or to each impeller spindle. In a reversible pump, a thrust surface can be provided axially adjacent to both end faces of the impeller spindle, with the impeller spindle then being accommodated with axial clearance between the two thrust surfaces.

[0010] As described, the spindle package is hydraulically synchronized, meaning it adjusts automatically during operation. In particular, there is no or negligible mechanical force transmission between the drive spindle and the idler spindles, resulting in minimal axial offset of the idler spindles during operation. Therefore, the axial play between the idler spindle and the axial thrust surfaces can be designed to be correspondingly small, naturally depending on the given size of the screw spindle pump. The axial play is preferably between 0.3 mm for small screw spindle pumps and 5.0 mm for very large screw spindle pumps; the play is preferably in the range between 1.0 and 3.0 mm. The play is designed with regard to the given axial offset of the idler spindles, whereby the specified values ​​​​represent the total play that the respective idler spindle has between the two thrust washers.

[0011] Various options are available for the implementation of the thrust surfaces. For example, the thrust surface, or each thrust surface, can be formed by means of a coating on the housing. In this case, one or more corresponding housing shoulders are provided on the housing side, forming the basis for the thrust surface, which is implemented by means of a coating on the housing shoulder. Alternatively, the thrust surface, or each thrust surface, can also be implemented by means of a thrust washer. In this case, a specific thrust washer is inserted into the housing at a specific position to create the thrust surface. The clearance can be adjusted very precisely using the thickness of the thrust washer.

[0012] Extremely low-wear surfaces are preferably provided as contact surfaces, which means that a correspondingly low-wear material is used. Coatings or thrust washers made of a ceramic or carbide material or of a composite material containing a ceramic or carbide material are suitable for this. This means that, in principle, a technical ceramic is used, which can optionally be reinforced with glass or carbon fibers. A ceramic material or a technical ceramic based on silicon is expediently used, with SiC or Si 3 N 4 being particularly suitable for this, or WC, which material can also be fiber-reinforced if required, as described. The use of Cr 2 O 3 is also conceivable. Alternatively, a hard metal can be used to form the coating, just as the thrust washer(s) can be made of hard metal or can have hardened surfaces.The hardness should be at least 1000 HV. The thrust surfaces, coatings, or thrust washers are therefore not prone to wear, similar to the steel spindles, which are preferably Kolsterized or cold nitrided and, as described, ideally have plain bearings via the hydrostatic lubricating film on the thrust surfaces, coatings, or thrust washers.

[0013] The or each spindle is received in a corresponding spindle bore, which overlaps with a drive spindle bore accommodating the drive spindle. The one or both spindle bores are axially delimited by one or two axial housing shoulders, on which housing shoulders the respective thrust surface is formed or thrust washer is supported. Accordingly, defined shoulders are provided in the housing, which serve either as supports for the coating(s) or as axial support points for the thrust washers. Thus, either the coating is applied directly to such a housing shoulder, or a thrust washer rests against such a housing shoulder.The axial distance between two housing shoulders can be defined and adjusted very precisely, so that defined geometric relationships are achieved and the axial play of the respective running spindle can also be adjusted precisely when using separate thrust washers by selecting the appropriate thrust washer thickness.

[0014] As described, the drive spindle is hydraulically balanced in such a way that no significant axial force acts on the drive spindle which would push the drive spindle in one direction and which would then in turn lead to a displacement of the running spindles. The running spindles, which are mounted in a quasi-axially cantilevered manner, are moved slightly axially solely by the pressure building up in the pump housing, utilizing the axial play, as far as the profile engagement allows. The drive spindles and the running spindles are accommodated in a pump chamber which is sealed to one drive side of the drive spindle by a sealing element, preferably a single sealing element which seals between the drive spindle and the housing. This means that the pump chamber is sealed to one side by just one sealing element. According to the invention, this sealing element is selected or designed in such a way with regard to the dimensions and geometry of the drive spindle.designed so that the axially compressed surface of the sealing element essentially corresponds to the axially compressed surface of the drive spindle or the drive spindle profile. This provides the axial hydraulic compensation, which has proven particularly advantageous with regard to hydraulic synchronization of the spindle package and minimizing the resulting axial spindle offset during operation. The compressed surface of the annular sealing element through which the drive spindle passes ultimately corresponds to its axial annular surface facing the pump chamber. As is well known, the compressed surface of the drive spindle, viewed in the longitudinal direction of the spindle, is made up of several, sometimes sickle-shaped surface sections of the spindle profile protruding radially from the spindle core, resulting from the engagement of the spindle profile in the two spindle profiles of the spindles.The difference between the two pressed surfaces should be a maximum of 10%, preferably only a maximum of 5%, and ideally of course zero, so that only a very small resulting axial force, if any, occurs, which causes neither an axial offset of the drive spindle nor a significant load on the spindle bearing.

[0015] The sealing element itself is preferably a mechanical seal which is arranged on the drive spindle and seals against a corresponding sealing section or sealing seat on the housing.

[0016] The drive spindle itself is expediently mounted radially in the housing on only one side, outside a pump chamber containing the working and running spindles, from which a section of the drive spindle extends. A radial bearing is expediently used for this purpose, with only a single radial bearing being used preferably. This can be a single-row or multi-row bearing in the form of a ball, roller, or spherical bearing, etc., i.e. a rolling bearing. Due to the hydraulic balancing of the drive spindle on the one hand and the arrangement of two running spindles, which are offset by 180° next to the drive spindle, it is possible to use only a single radial bearing, since this is almost load-free during operation due to the corresponding force balancing.

[0017] A useful development of the invention provides that at least the or each spindle bore is lined with a sliding lining, wherein the spindles are arranged with radial clearance relative to the sliding lining. Lining the spindle bores with the sliding lining also serves to reduce any dead space. This is because, during normal operation, no radial movement of the spindles occurs; rather, a thin hydraulic lubricating film forms between the spindle surface and the spindle bores or the sliding lining, which also supports the spindle. This, in turn, makes it possible to minimize the radial spindle play via the sliding lining and thus also reduce the dead space accordingly.

[0018] A plastic coating is advantageously used as such a sliding coating, in particular one made of hydrogenated acrylonitrile butadiene rubber (HNBR), chlorotrifluoroethylene, an ethylene propylene diene (monomer) rubber (EPDM), polytetrafluoroethylene (PTFE), a perfluoroalkoxy polymer, a fluororubber (FKM), or a perfluororubber (FFKM). However, this list is not exhaustive; rather, other suitable plastic materials may also be used, as long as they are suitable for the medium to be pumped.

[0019] The thickness of the sliding lining is preferably adjusted so that the radial clearance is between 0.01 and 1.00 mm, especially between 0.05 and 0.5 mm. This means that the radial clearance is extremely low, resulting precisely from the fact that no significant radial movement occurs during operation.

[0020] In addition to the dead-space-optimized design of the screw pump, the design of the screw pump with regard to minimizing both axial and radial clearance also has the advantage of significantly increasing the efficiency of the screw pump compared to conventional screw pumps, by up to several tens of percent. This is because the minimal clearances ensure a consistent volume flow across a wide pressure range, with hardly any backflow of the pumped medium occurring due to the minimal gaps. This means that significantly more efficient pumping operation can be achieved, combined with a screw pump design that is extremely advantageous from a hygienic perspective.

[0021] As described, the screw pump is used in particular for conveying critical substances whose processing requires the highest level of cleanliness. Accordingly, the screw pump according to the invention is used for conveying viscous or pasty foods, pharmaceutical, cosmetic, or chemical agents. Viscous or pasty foods can be, for example, dairy products such as cream cheese, cream, quark, butter, or yogurt. The conveyance of significantly more viscous or pasty foods such as ketchup, mayonnaise, mustard, and the like, horseradish, processed cheese, vegetable oils, liquid egg, dough, or fruit puree is also conceivable, as are gelatin, syrup, nut or nougat creams, chocolate, honey, marzipan, or other fats or oils. In the pharmaceutical and cosmetic sectors, pumpable media include, for example, liquid soaps, creams, lotions, or similar.In the chemical sector, examples include liquid detergents, dishwashing liquids, cleaning agents, as well as paints and similar materials. This list is, of course, not exhaustive, but it demonstrates that the viscosity of the materials that can be pumped covers an extremely wide range. The viscosity of the materials that can be pumped using the screw spindle pump according to the invention is in the range between 0.5 and 1 million kg•m -1< •s -1< .

[0022] Further advantages and details of the invention will become apparent from the following exemplary embodiment and the drawings. These show: Fig. 1 is a schematic diagram of a screw spindle pump according to the invention of a first embodiment, in quarter section, and Fig. 2 is a schematic diagram of a screw spindle pump according to the invention of a second embodiment, in quarter section.

[0023] The Figur 1 shows, partially in section, a screw pump 1 according to the invention, comprising a housing 2, which here consists exemplarily of four housing parts 2a, 2b, 2c and 2d. The housing is therefore of modular construction. A pump chamber 3 is formed inside the housing, with an axial inlet 4 and a radial inlet 5. The delivery direction of the screw pump 1 is reversible, i.e., depending on the delivery direction, the inlet 4 can be the suction connection and the inlet 5 the pressure connection, or vice versa. Although an axial inlet 5 and a radial inlet 4 are shown here, the access configuration can also be different, e.g. with two radial inlets, which can also be offset around the longitudinal axis of the housing.

[0024] In addition to the pressure chamber 3, the housing 2 also has a bearing chamber 6 in which, as will be described below, a drive spindle is mounted.

[0025] The screw spindle pump 1 further comprises a spindle assembly comprising a centrally arranged drive spindle 7 with a drive spindle profile 8 and two laterally adjacent and offset by 180° from each other running spindles 9, each with a running spindle profile 10, wherein the drive spindle profile 8 meshes with the running spindle profiles 10. In the example, two running spindles 9 are shown; alternatively, only one running spindle 9 or three running spindles 9 can be provided.

[0026] The drive spindle 7 or the drive spindle profile 8 is received in a corresponding drive spindle bore (not shown in detail here) in the housing 2 or in the housing part 2b, respectively, while the two running spindles 9 are received in corresponding running spindle bores 11 in the housing 2 or in the housing part 2b, respectively. The two running spindle bores 11 overlap with the drive spindle bore 9 in a known manner, whereby the bores form a substantial part of the pump chamber 3.

[0027] The two housing parts 2a and 2c have corresponding housing shoulders 12 in the area of ​​the two idler spindle bores 11, which each serve as support surfaces for a thrust washer 13, which are axially spaced from one another and each accommodate a idler spindle 9 between them. Each thrust washer 13 forms a thrust surface for the end face of the axially adjacent idler spindle 9 or has such a thrust surface. They are flat on both sides, i.e. they lie flat against the corresponding housing shoulders 12, just as they run parallel to the corresponding flat end faces of the idler spindles 9. Each idler spindle 9 is accommodated between the two thrust washers 13 with slight axial play, depending on the size of the screw spindle pump between 0.3 - 5.0 mm, in particular between 1.0 - 3.0 mm, and can therefore be slightly moved axially.The maximum axial play is adjusted via the thickness of the thrust washers 13 used, so that it can be minimized and the dead space there can be minimized.

[0028] The thrust washers 13 are, for example, washers made of a ceramic material or of a composite material containing a ceramic material, preferably of a technical ceramic. A silicon-based material, in particular SiC or Si 3 N 4, is preferably used. Alternatively, each thrust washer 13 can also be made of a carbide material, e.g. WC. The use of thrust washers 13 made of hard metal is also conceivable. These are therefore extremely low-wear thrust washers 13, and the respective running spindle 9, which is made of a suitable stainless steel, e.g. kolsterized or cold nitrided, is also correspondingly wear-resistant. Both the spindles and the housing are made of a stainless steel that is particularly suitable for use in the food industry, the medical, pharmaceutical and chemical industries.

[0029] The drive spindle 7, as the partially sectioned figure shows, is led from the pump chamber 3 into the bearing chamber 6, where it is mounted in the housing 2 via a radial bearing 14, a rolling bearing preferably in the form of a single or multi-row ball bearing or a roller or spherical bearing. This provides the rotary support of the drive spindle 7 in a single bearing plane. A single such bearing plane is sufficient because, as will be discussed below, the drive spindle 7 is axially hydraulically balanced, meaning that no or only a negligible axial force acts on the drive spindle 7 during pump operation, as well as no or only a negligible radial force resulting from the symmetrical bilateral arrangement of the two running spindles 7, which in turn are hydraulically supported or mounted via a sliding film, both axially and radially, as will be discussed below.

[0030] Furthermore, a single sealing element 15 is provided, which is preferably a radial mechanical seal, which is arranged on the drive spindle 7 and seals against a corresponding sealing seat in the housing 2. This single spindle seal or sealing plane seals the entire pump chamber 3 on this side, i.e., the drive side. This means that the fluid or medium can only flow from inlet 4 to inlet 5 or vice versa; passage to the bearing side or drive side (the actual pump drive is connected to the corresponding end-side impeller spindle connection) is excluded.

[0031] As described, the drive spindle 7 is axially hydraulically balanced so that no or only a completely negligible axial force acts on the drive spindle 7. This is achieved by designing the sealing element 15 accordingly with regard to the drive spindle profile 8. The design is such that the surface of the sealing element 15 subjected to pressure by the medium, i.e., the surface facing the pump chamber 3, is essentially equal to the axially compressed surface of the work spindle profile 8. The axially compressed surface of the work spindle profile 8, viewed in the longitudinal direction of the spindle, results, in a known manner, from the meshing engagement of the work spindle profile 8 with the running spindle profiles 10, from several, partially sickle-shaped surface sections of the work spindle profile 7, which add up to a total surface.This total surface area is now almost, or ideally, completely the same size as the axially compressed annular surface of the sealing element 15 facing the pump chamber. Any surface difference should be a maximum of 10%, preferably a maximum of 5%. The pressure acting on the respective surfaces is directed opposite to each other, so that, since both surfaces are subjected to the same pressure, ideally complete pressure equalization results. Consequently, the drive spindle 7 is virtually pressure-free or hydraulically balanced, so that ideally no or only a negligible axial force acts on it.

[0032] This results in no mechanical power transmission from the drive spindle 7 to the two spindles, as this spindle is axially fixed during operation. Only a slight axial displacement of the spindles 9 occurs due to the working pressure, which leads to a slight axial movement of the spindles 9 in the spindle bores 11 and to the corresponding end face of the respective spindle 9 coming into contact with the respective thrust washer 13. The two opposing surfaces are preferably hydrostatically supported by a thin lubricating film of the medium to be pumped, so that no wear occurs in this area.

[0033] To further minimize dead space and improve efficiency by minimizing medium backflow through given gaps, the respective spindle bore 11 is provided with a sliding coating 16, which is preferably a sliding coating 16 made of a plastic such as HNBR, EPDM, PTFE, CTFE, PFA, FKM, or FFKM. The thickness of the sliding coating 16 is selected such that only minimal radial play results between the respective spindle 9, i.e., its outer surface, and the sliding coating 16, whereby this radial play should be between 0.01 - 1.0 mm, in particular between 0.05 - 0.5 mm. This means that only minimal play is present here, so that any backflow can be minimized, combined with an improvement in efficiency.Here, too, a corresponding medium lubricating film is formed, via which the running spindles 9 are virtually slidingly mounted towards the sliding lining 16, so that here, too, no abrasion occurs.

[0034] During operation, the drive spindle 7 is driven by the drive in a known manner; it rotates. The profile engagement inevitably causes the running spindles 9 to rotate and the medium is conveyed accordingly from inlet 4 to inlet 5 or vice versa, i.e. from the suction port to the pressure port, depending on the direction of rotation of the drive spindle 7. During start-up, the two running spindles 9 are minimally axially displaced, as described, by the build-up of working pressure and the resulting minimal axial play within the profile engagement; they each run against one of the thrust washers 13, where they are preferably slide-mounted via the resulting sliding film from the medium to be conveyed. Due to the minimal gap, there is only extremely low backflow, which leads to improved efficiency.

[0035] The embodiment of the screw pump 1 according to Fig. 2 The basic structure corresponds to that of Fig. 1 Here, too, a modular housing 2 is provided, comprising, for example, three housing parts 2a, 2b, 2c, and 2d. A pump chamber 3 is formed inside the housing, with an axial access 4 and a radial access 5. The delivery direction of the screw pump 1 is also reversible here. In addition to the pressure chamber 3, the housing 2 also has a bearing chamber 6, in which, as described below, a drive spindle is mounted.

[0036] The screw spindle pump 1 also comprises a spindle package with a central drive spindle 7 with a drive spindle profile 8 and two laterally adjacent running spindles 9 arranged at a 180° angle to each other, each with a running spindle profile 10, wherein the drive spindle profile 8 meshes with the running spindle profiles 10. In the example, two running spindles 9 are shown; alternatively, only one running spindle 9 or three running spindles 9 can be provided.

[0037] The drive spindle 7 is received in a corresponding drive spindle bore in the housing 2, while the two idler spindles 9 are received in corresponding idler spindle bores 11 in the housing 2. The two idler spindle bores 11 overlap with the drive spindle bore 9 in a known manner, with the bores in turn forming an essential part of the pump chamber 3.

[0038] The two housing parts 2a and 2c have corresponding housing shoulders 12 in the area of ​​the two running spindle bores 11. The housing shoulders 12 are axially spaced from one another. A running spindle 9 is received between them. Each housing shoulder 12 is provided with a coating 17 which forms a contact surface for the end face of the axially adjacent running spindle 9. The coating 17 is made of, for example, Si 3 N 4 , SiC, WC or Cr 2 O 3 and is applied directly to the respective housing shoulder 12. The end faces of the running spindles 9 are flat, i.e. they lie flat with or against the corresponding coatings 17 of the housing shoulders 12. Each running spindle 9 is accommodated with a slight axial play, depending on the size of the screw spindle pump between 0.3 - 5.0 mm, in particular between 1.0 - 3.0 mm, between the two housing shoulders 12 or the low-wear coatings 17, and can therefore be slightly displaced axially.During operation, the spindles 9 also run against the contact surfaces or coatings 17 and are ideally supported there by a hydraulic lubricating film or are supported by plain bearings. In any case, the coatings, as well as the spindles themselves, are extremely wear-resistant, ensuring continuous operation.

[0039] Here, the contact surfaces are realized directly on the housing itself by means of the coatings 17. The arrangement of the separate contact washers, as in the embodiment according to Fig. 1 , is not required here. Nevertheless, the same advantages as in the example according to Fig. 1 described.

[0040] Otherwise, the structure corresponds to that in Fig. 2 shown screw pump 1 the example from Fig. 1 , ie that here too a radial bearing 15 is provided for supporting the drive spindle 7, as well as at least the spindle bores 11 are covered with a sliding coating 16. The statements regarding the pump from Fig. 1 is therefore referred to, they apply equally to the pump according to Fig. 2 .

[0041] In the event that the screw spindle pump 1 is not reversible, only one thrust washer 13 or coating 17 forming the contact surface is provided per impeller spindle bore 11, namely at the suction-side end of the respective impeller spindle bore, after the impeller spindle 9 moves minimally towards the suction side during operation.

[0042] The screw pump according to the invention is clearly designed simply, as it does not require a device for hydraulic thrust compensation of the impeller spindles 7, which is disadvantageous when conveying food or other hygienically sensitive media. Rather, the design of the screw pump allows it to be cleaned in the assembled state, since there are no other volumes other than the pump chamber in which the medium to be pumped can be contained. This enables easy flushing of the screw pump in the installed state, i.e., "cleaning in place." By integrating the thrust washers 13, the permissible axial play of the impeller spindles 9 can be minimized. As mentioned, a direct disk contact occurs, thus eliminating any disadvantageous dead space in this pump chamber area.

[0043] The use of three spindles—namely, the drive spindle 7 and the two idler spindles 9—enables a more pressure-resistant delivery curve, as the screw spindle pump 1 has a very dense profile. This enables applications with high dosing accuracy. The dense profile also results in better suction behavior, resulting in improved efficiency. Furthermore, the screw spindle pump 1, or rather the spindle assembly, is hydraulically synchronized, meaning it adjusts itself during operation, with no mechanical power transmission between the drive spindle 7 and the idler spindles 9.

Claims

1. Screw spindle pump comprising at least one running spindle without hydraulic axial thrust balancing, comprising a housing (2) with a pump chamber (3) and also comprising a drive spindle (7), which is accommodated therein, as well as the at least one running spindle (9), which meshes with the drive spindle and has in each case two end surfaces, characterized in that the drive spindle (7) is hydraulically balanced in such a way that, during pump operation, only a negligible axial force, if any at all, acts on the drive spindle (7), wherein the pump chamber (3) is sealed in the direction of a drive side of the drive spindle via a sealing element (15) between the drive spindle (7) and the housing (2), the sealing element being arranged on the drive spindle, wherein that surface of the sealing element (15) which faces the pump chamber (3), and is subjected to axial pressure by a delivered medium, is essentially equal to the size of the surface of a drive-spindle profile (8) that is subjected to axial pressure by the medium, which drive-spindle profile (8) protrudes radially from a spindle core, and in that a stop surface (13) is provided axially adjacent to at least one end surface of the running spindle (9), wherein the running spindle (9) is accommodated with axial play perpendicularly to the stop surface (13) and such that it can be displaced against the stop surface (13).

2. Screw spindle pump according to Claim 1, characterized in that a stop surface is provided axially adjacent to the two end surfaces of the running spindle, wherein the running spindle (9) is accommodated with axial play between the two stop surfaces.

3. Screw spindle pump according to Claim 1 or 2, characterized in that the axial play is between 0.3 - 5.0 mm, in particular between 1.0 - 3.0 mm.

4. Screw spindle pump according to one of the preceding claims, characterized in that the or each stop surface is formed by means of a coating on the housing, or in that the or each stop surface is realized by means of a stop disc.

5. Screw spindle pump according to Claim 4, characterized in that the or each coating or the or each stop disc consists of a ceramic or carbide material or of a hard metal or of a composite containing ceramic or carbide material.

6. Screw spindle pump according to Claim 4 or 5, characterized in that the or each coating or the or each stop disc is made of a silicon-based material, in particular SiC or Si3N4, or is made of WC or of Cr2O3.

7. Screw spindle pump according to one of the preceding claims, characterized in that the or each running spindle (9) is accommodated in a running-spindle bore (11), which overlaps with a drive-spindle bore accommodating the drive spindle (7), wherein the running-spindle bores (11) are delimited axially via one or two axial housing shoulders (12), on which housing shoulder or shoulders (12) the stop surface is formed or the stop disc (13) is supported.

8. Screw spindle pump according to one of the preceding claims, characterized in that the sealing element (15) is a mechanical seal.

9. Screw spindle pump according to one of the preceding claims, characterized in that the sealing element (15) is arranged on the drive spindle (7) and provides sealing in the direction of a sealing portion on the housing (2).

10. Screw spindle pump according to one of the preceding claims, characterized in that the drive spindle (7) is rotatably mounted radially in the housing (2) only on one side outside a pump chamber (3), which comprises the drive spindle and running spindle (7, 9) and out of which a portion of the drive spindle (7) is led.

11. Screw spindle pump according to Claim 10, characterized in that the radial rotatable mounting is realized via a radial bearing (14), preferably just one radial bearing.

12. Screw spindle pump according to one of the preceding claims, characterized in that at least the or each running-spindle bore (11) is lined with a sliding coating (16), wherein the running spindles (7) are arranged with radial play in relation to the sliding coating (16).

13. Screw spindle pump according to Claim 12, characterized in that, forming a sliding coating (16), a plastic coating consists in particular of a hydrogenated acrylonitrile butadiene rubber, chlorotrifluoroethene, an ethylene propylene diene (monomer) rubber, polytetrafluoroethylene, a perfluoroalkoxy polymer, a fluorinated rubber or a perfluorinated rubber.

14. Screw spindle pump according to Claim 12 or 13, characterized in that the radial play is between 0.01 - 1.0 mm, in particular between 0.05 - 0.5 mm.

15. Use of a screw spindle pump (1) according to one of the preceding claims for delivering viscous or pasty foodstuffs, or pharmaceutical, cosmetic or chemical media.