EP3507497B1 - Vacuum pump screw rotor - Google Patents

Description

The invention relates to a vacuum pump screw rotor.

Screw vacuum pumps, such as in EP1 890 039 described, have two rotor elements in a pumping chamber formed by a housing. The rotor elements have a helical contour and are rotated in opposite directions to convey gases. In order to achieve a high internal compression, i.e. a volume ratio between the inlet and the outlet of the pump, it is known that the displacement element of the rotor element, i.e. the helical contour, has a changing pitch. On the inlet side or suction side, the pitch is large and the volume of the chambers formed per turn is also large. The pitch decreases in the direction of the outlet, so that on the outlet or pressure side there is a small pitch and also small chamber volumes per turn. By providing a variable pitch, low power consumption at low inlet pressures and at the same time low thermal load on the pump can be achieved. Providing a changing pitch requires a complex and therefore expensive manufacturing process. In particular, the manufacturing process such as milling or turning the turns, i.e. the helical recesses, must be carried out in several successive work steps.

The object of the invention is to create a vacuum pump screw rotor that can be produced cost-effectively with low power consumption and low thermal load on the pump. Furthermore, the object of the invention is to create a corresponding screw vacuum pump and a suitable manufacturing method.

The object is achieved according to the invention by a vacuum pump screw rotor according to claim 1, a vacuum pump according to claim 10, and a manufacturing method according to claim 14.

The vacuum pump screw rotor according to the invention has at least two helical displacement elements arranged on a rotor shaft. The rotor element is formed by the displacement elements. According to the invention, the at least two displacement elements have different pitches, with the pitch being constant for each displacement element. For example, the vacuum pump screw rotor according to the invention has two displacement elements, with a first suction-side displacement element having a larger constant pitch and a second pressure-side displacement element having a smaller constant pitch. By providing several displacement elements according to the invention, each of which has a constant pitch, production is considerably simplified.

According to the invention, each displacement element has at least one helical recess which has the same contour over its entire length. The contours are preferably different for each displacement element. The individual displacement element therefore preferably has a constant pitch and a consistent contour. This simplifies production considerably, so that production costs can be greatly reduced.

To further improve the suction performance, the contour of the helical recess of the suction-side displacement element, i.e. in particular the first displacement element in the pumping direction, is asymmetrical. Due to the asymmetrical design of the contour or profile, the flanks can be designed in such a way that the leakage areas, the so-called blow holes, in particular disappear completely or at least have a small cross-section. A particularly suitable asymmetrical profile is the so-called "Quimby profile". Although such a profile is relatively difficult to produce, it has the advantage that there is no continuous blow hole. A short circuit only occurs between two adjacent chambers. Since this is an asymmetrical profile with different profile flanks, at least two work steps are required for production, since the two flanks have to be produced in different work steps due to their asymmetry.

The pressure-side displacement element, in particular the last displacement element in the pumping direction, is provided with a symmetrical contour of the helical recess.

The symmetrical contour has the particular advantage that it is easier to manufacture. In particular, both flanks with a symmetrical contour can be manufactured in one work step using a rotating end mill or a rotating disk mill. Such symmetrical profiles only have small blow holes, but these are continuous, i.e. not just provided between two adjacent chambers. The size of the blow hole decreases when the pitch is reduced. In this respect, such symmetrical profiles can be provided in particular in the pressure-side displacement element, since in a preferred embodiment this has a smaller pitch than the suction-side displacement element and preferably also than the displacement elements arranged between the suction-side and pressure-side displacement elements. Although the tightness of such symmetrical profiles is somewhat lower, they have the advantage that they are significantly easier to manufacture. In particular, it is possible to produce the symmetrical profile in a single work step and preferably with a simple end mill or disc mill. This reduces costs considerably. A particularly suitable symmetrical profile is the so-called "cycloid profile".

The provision of at least two such displacement elements means that the corresponding screw vacuum pump can generate low inlet pressures with low power consumption. The thermal load is also low. The arrangement of at least two displacement elements designed according to the invention with a constant pitch and consistent contour in a vacuum pump leads to essentially the same results as with a vacuum pump with a displacement element with a changing pitch. With high installed volume ratios, three or four displacement elements can be provided per rotor.

In order to reduce the achievable inlet pressure and/or to reduce the power consumption and/or the thermal load, in a particularly preferred embodiment, a pressure-side displacement element, i.e. in particular the last displacement element in the pumping direction, has a large number of turns. A large number of turns means that a larger gap between the screw rotor and the housing can be accepted while maintaining the same performance. The gap can have a cold gap width of 0.1 - 0.3 mm. A large number of outlet turns or number of turns in the pressure-side displacement element can be produced inexpensively because, according to the invention, this displacement element has a constant pitch and in particular also a symmetrical contour. This enables simple and inexpensive production, so that the provision of a larger number of turns is acceptable. This pressure-side or last displacement element preferably has more than 8, in particular more than 10 and particularly preferably more than 12 turns. The use of symmetrical profiles has the advantage in a particularly preferred embodiment that both flanks of the profile can be cut simultaneously with one milling cutter. In this case, the Milling cutter through the opposite flank, so that deformation or bending of the milling cutter during the milling process and the resulting inaccuracies are avoided.

To further reduce manufacturing costs, it is particularly preferred to form the displacement elements and the rotor shaft in one piece.

In a further preferred embodiment, the change in pitch between adjacent displacement elements is discontinuous or abrupt. If necessary, the two displacement elements are arranged at a distance from one another in the longitudinal direction, so that a circumferential cylindrical ring-shaped chamber is formed between two displacement elements, which serves as a tool outlet. This is particularly advantageous in the case of rotors formed in one piece, since the tool producing the helical line can be easily guided out in this area. If the displacement elements are manufactured independently of one another and then mounted on a shaft, the provision of a tool outlet, in particular such an annular cylindrical area, is not necessary.

In a preferred development of the invention, no tool outlet is provided between two adjacent displacement elements at the pitch change. In the area of the pitch change, both flanks preferably have a defect or recess in order to be able to guide the tool out. Such a defect has no significant influence on the compression performance of the pump, since it is a very localized defect or recess.

The vacuum pump screw rotor according to the invention has in particular several displacement elements. These can each have the same or different diameters. It is preferred that the The pressure-side displacement element has a smaller diameter than the suction-side displacement element.

In the case of displacement elements manufactured independently of the rotor shaft, these are mounted on the shaft using press fits, for example. It is preferred to provide elements such as dowel pins to determine the angular position of the displacement elements relative to one another.

In particular, in the case of a one-piece design of the screw rotor, but also in the case of a multi-piece design, it is preferred to manufacture it from aluminum or an aluminum alloy. It is particularly preferred to manufacture the rotor from aluminum or an aluminum alloy, in particular AlSi7Mg or AISi17Cu4Mg. The alloy preferably has a high silicon content of preferably more than 15% in order to reduce the coefficient of expansion.

In a further preferred development of the invention, the aluminum used has a low coefficient of expansion. It is preferred if the material has a coefficient of expansion of less than 18 * 10 ^-6 / K. In a further preferred embodiment, the surface of the displacement elements is coated, in particular a coating against wear and/or corrosion is provided. In this case, it is preferred to provide an anodic or another suitable coating depending on the area of application.

The invention further relates to a screw vacuum pump. This has two intermeshing vacuum pump screw rotors as described above. The two screw rotors are arranged in a suction chamber formed by a pump housing. Usually, one of the two screw rotors is connected to a drive device such as an electric motor. The two screw rotors can be connected to one another via gears, which are arranged in particular on the rotor shafts. This also results in synchronization of the counter-rotating screw rotors. In a particularly preferred embodiment, it is possible, in particular due to the inventive design of the screw rotors, to achieve an internal compression of the screw vacuum pump of at least two, in particular at least four. Such a high internal compression is possible in particular due to the design of the two rotors with at least two compression elements, each with a constant pitch and in particular also a constant contour, with a high number of turns of the pressure-side displacement element. This is particularly possible although large gaps are permitted in the area of the pressure-side displacement element. The large gaps have the particular advantage that the thermal load is distributed more evenly over the pressure-side displacement element. In particular, the thermal expansion of the corresponding displacement element and thus the risk of the displacement element touching the inside of the housing is also avoided. Another aspect in this regard is that the screw rotors have a lower expansion coefficient than the housing. In particular, the expansion coefficient of the housing is at least 5%, and particularly preferably at least 10% greater than that of the screw rotors.

It is preferred that the housing is made of an aluminum alloy with a lower silicon content than the silicon content in the material of the screw rotors. This ensures a higher thermal expansion of the housing compared to the screw rotors. This ensures in particular that during operation, i.e. with increasing thermal load, the gap can become smaller, but that a sufficient gap always remains between the outside of the displacement elements and the inside of the suction chamber.

Furthermore, the invention relates to a method for producing a screw rotor as described above. The production takes place in particular such that the displacement elements and the rotor shaft are formed in one piece. In a first step, a base body for the screw rotor is provided. The helical recesses for producing the displacement element are produced using a form milling cutter such as a finger milling cutter or disc milling cutter. This is done in a separate step for each displacement element, since the pitch and in particular the contour of the helical recesses are different for each displacement element.

It is preferred that in displacement elements with a symmetrical contour the recess is made with a single tool, in particular in a single operation. Furthermore, it is preferred that the tool reproduces the outer contour of the recess so that the production of preferably both flanks can be carried out in one work step. In the case of the asymmetrical part, the flanks must be machined by two different tools.

It is preferred that, particularly in the case of screw rotors manufactured in one piece, a tool run-out is produced before the helical recesses are produced. Such a ring-cylindrical recess can be produced by milling or turning.

In a particularly preferred development, no such tool run-out is provided. Instead, a recess or defect is provided in a flank of an adjacent displacement element. The defect or recess is created when the milling cutter is guided out.

The base body used is in particular cylindrical, so that the rotor shaft with any shaft journals connected to it and in particular the displacement elements can be manufactured from a single base body. It is also possible to use a base body that is designed as a semi-finished product and already has recesses and/or bearing journals. The base body can be manufactured using a casting process, for example.

The invention is explained in more detail below using a preferred embodiment with reference to the accompanying drawings.

Fig. 1

eine schematische Draufsicht einer ersten bevorzugten Ausführungsform eines Vakuumpumpen-Schraubenrotors,

Fig. 2

eine schematische Draufsicht einer zweiten bevorzugten Ausführungsform eines Vakuumpumpen-Schraubenrotors,

Fig. 3

eine schematische Schnittansicht von Verdrängungselementen mit asymmetrischem Profil,

Fig. 4

eine schematische Schnittansicht von Verdrängungselementen mit symmetrischem Profil und

Fig. 5

eine schematische Schnittansicht einer Schrauben-Vakuumpumpe.

Show it:

Fig.1

a schematic plan view of a first preferred embodiment of a vacuum pump screw rotor,

Fig.2

a schematic plan view of a second preferred embodiment of a vacuum pump screw rotor,

Fig.3

a schematic sectional view of displacement elements with asymmetric profile,

Fig.4

a schematic sectional view of displacement elements with symmetrical profile and

Fig.5

a schematic sectional view of a screw vacuum pump.

In the first preferred embodiment of the vacuum pump screw rotor, the rotor has two displacement elements 10, 12. A first suction-side displacement element 10 has a large pitch of approximately 50 - 150 mm/revolution. The pitch is constant over the entire displacement element 10. The contour of the helical recess is also constant. The second pressure-side displacement element 12 has a constant pitch and a constant contour of the recess over its length. The pitch of the pressure-side displacement element 12 is preferably in the range of 10 - 30 mm/revolution. Between A ring-cylindrical recess 14 is provided between the two displacement elements. This serves to ensure that the one-piece design of the Fig.1 A tool run-out is realized in the screw rotor shown.

Furthermore, the one-piece screw rotor has two bearing seats 16 and a shaft end 18. A gear wheel for driving, for example, is connected to the shaft end 18.

At the Fig.2 In the second preferred embodiment shown, the two displacement elements 10, 12 are manufactured separately and then fixed to a rotor shaft 20, for example by pressing. This manufacture is somewhat more complex, but the cylindrical distance 14 between two adjacent displacement elements 10, 12 is not required as a tool run-out. The bearing seats 16 and the shaft ends 18 can be an integral part of the displacement elements. Alternatively, a continuous shaft 20 can be manufactured from a different material than the displacement elements 10, 12.

Fig.3 shows a schematic sectional view of an asymmetrical profile (e.g. a Quimby profile). The asymmetrical profile shown is a so-called "Quimby profile". The sectional view shows two screw rotors that mesh with each other and whose longitudinal direction is perpendicular to the plane of the drawing. The opposing rotation of the rotors is indicated by the two arrows 15. In relation to a plane 17 running perpendicular to the longitudinal axis of the displacement elements, the profiles of the flanks 19 and 21 are designed differently for each rotor. The opposing flanks 19, 21 must therefore be manufactured independently of each other. Although this is somewhat more complex and difficult to manufacture, it has the advantage that there is no continuous blow hole, but rather just a short circuit between two adjacent chambers.

Such an asymmetrical profile is preferably provided in the suction-side displacement element 10.

The schematic sectional view in Fig.4 shows a cross-section of two displacement elements or two screw rotors, which in turn rotate in opposite directions (arrows 15). In relation to the axis of symmetry 17, the flanks 23 of each displacement element are symmetrical. In the Fig.4 The preferred embodiment of a symmetrically designed contour shown is a cycloid profile.

A symmetrical profile, as in Fig.4 shown, is preferably provided on the pressure-side displacement elements 12.

In the further Fig.5 The embodiment shown is again a one-piece design. A recess or defect is provided in the flank of the displacement element 12 to guide the tool, such as a milling cutter, out.

It is also possible for more than two displacement elements to be provided. These can also have different head diameters and corresponding foot diameters. It is preferred that a displacement element with a larger head diameter is arranged at the inlet, i.e. on the suction side, in order to achieve a greater suction capacity in this area and/or to increase the built-in volume ratio. Combinations of the embodiments described above are also possible. For example, one or more displacement elements can be manufactured in one piece with the shaft or an additional displacement element can be manufactured independently of the shaft and then mounted on the shaft.

A schematic sectional view of a vacuum pump ( Fig.5 ) shows two vacuum pump screw rotors 26 arranged in a suction chamber 24 in a housing 22. The two rotors are mounted in the housing via bearings 28. Gears 32 are connected to each of the two shaft ends 18. These mesh with each other so that synchronization of the two shafts is ensured. One of the two gears 32 is connected to a drive device such as an electric motor.

As from Fig.5 As can be seen, the gas is sucked in in the area of the suction-side displacement elements 10, as shown by an arrow 34. The gas is expelled accordingly as shown by an arrow 36 at the end of the second, pressure-side displacement element 12.

Claims (19)

1. A vacuum pump screw rotor, comprising

   at least two helical displacer elements (10, 12) arranged on a rotor shaft, wherein the at least two displacer elements (10, 12) have pitches differing from each other but being constant for each displacer element, and

   wherein the displacer elements (10, 12) each comprise at least one helical recess, each recess having a uniform contour over its entire length,

   characterized in

   that the helical recess of a suction-side displacer element (10) has an asymmetric contour, and

   that the helical recess of a pressure-side displacer element (12) has a symmetric contour.
2. The vacuum pump screw rotor according to claim 1, characterized in that at least two rotor elements comprising respective helical displacer elements are provided, wherein the displacer elements have pitches differing from each other but being constant for each displacer element.
3. The vacuum pump screw rotor according to claim 1 or 2, characterized in that the pressure-side displacer element (10) comprises more than 8, preferably more than 10 and with particular preference more than 12 windings.
4. The vacuum pump screw rotor according to any one of claims 1 to 3, characterized in that a pressure-side displacer element is of the single-threaded type.
5. The vacuum pump screw rotor according to any one of claims 1 to 4, characterized in that the rotor shaft and the displacer elements (10, 12) are of a one-pieced design.
6. The vacuum pump screw rotor according to any one of claims 1 to 5, characterized in that the at least one change of pitch between two adjacent displacer elements (10, 12) is non-uniform (abrupt).
7. The vacuum pump screw rotor according to any one of claims 1 to 6, characterized in that the profile of the suction-side displacer element (10) is free of blowholes at least on one of the flanks.
8. The vacuum pump screw rotor according to any one of claims 1 to 7, characterized in that, between two displacer elements (10, 12), a tool run-out zone is provided at the change of pitch, and/or that between two displacer elements (10, 12) a void is provided at the change of pitch in at least one of the flanks of the displacer elements (10, 12).
9. The vacuum pump screw rotor according to any one of claims 1 to 8, characterized in that the entire vacuum pump screw rotor is made of aluminum or an aluminum alloy, particularly AlSi17Cu4Mg, wherein the aluminum has an expansion coefficient of less than 18*10^-61/K, and that particularly a high silicon percentage of at least 15% is provided.
10. A screw vacuum pump, comprising

    two mutually meshing screw rotors according to any one of claims 1 to 9,

    a housing (22) enclosing the screw rotors, and

    a drive means connected to the two screw rotors.
11. The screw vacuum pump according to claim 10, characterized in that the internal compression of the screw vacuum pump is at least 2, particularly at least 4.
12. The screw vacuum pump according to claim 10 or 11, characterized in that the screw rotors have a lower expansion coefficient than the housing (22), wherein the expansion coefficient of the housing (22) is particularly 5% and with particular preference 10% larger than that of the screw rotors, wherein the housing (22) is preferably made of aluminum or an aluminum alloy.
13. The screw vacuum pump according to any one of claims 10 to 12, characterized in that, between the pressure-side displacer elements and the housing, a gap is arranged, said gap having a height in the range of 0.05 mm to 0.5 mm, preferably 0.1 to 0.3 mm.
14. A method for producing a screw rotor according to any one of claims 1 to 9, comprising the steps of:

    - providing a base body of the screw rotor,

    - generating a helical recess of a first displacer element by use of a form cutter or a grinding screw, and

    - generating a further helical recess of a further displacer element by use of a further form cutter or grinding screw.
15. The method according to claim 14, characterized in that the manufacturing of displacer elements with symmetric profile is performed by use of a single tool, particularly in one working step.
16. The method according to claim 14 or 15, characterized in that, between adjacent displacer elements, prior to generating the helical recesses, a particularly circular cylindrical recess is generated as a tool run-out zone.
17. The method according to any one of claims 14 to 16, characterized in that, between two adjacent displacer elements, a recess is generated in at least one flank for withdrawal of the tool.
18. The method according to any one of claims 14 to 17, characterized in that, to generate the helical recesses, use is made, for each displacer element, of a tool reproducing the outer contour of the helical recess.
19. The method according to any one of claims 14 to 18, characterized in that the base body is cylindrical, wherein the base body is preferably formed as a semi-finished product with already partially preformed recess and/or bearing pin.

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