EP3507495B1 - Screw-type vacuum pump - Google Patents

Description

The invention relates to a screw vacuum pump.

Screw vacuum pumps have a pumping chamber in a housing, in which two screw rotors are arranged. Each screw rotor has at least one displacement element with a helical recess. As a result, a large number of turns are formed. In order to be able to achieve low pressures or a high vacuum of in particular less than 200 mbar (absolute pressure) with a low specific power consumption with the aid of screw vacuum pumps, known screw vacuum pumps have a high internal compression. The internal compression defines the reduction of the delivery volume from the inlet to the outlet of the pump. Low outlet pressures are achieved, in particular, in that a gap with a small height is formed between an outside of the at least one displacement element and an inside of the scooping chamber. In order to be able to realize such small gaps, reliable cooling of the screw rotors must be guaranteed. This is the only way to prevent the temperature of the rotor and thus of the at least one displacement element of the rotor, in particular in the pressure-side area of the screw vacuum pump, in which high pressure differences occur, from increasing in such a way that due to the expansion of the displacement element caused by the temperature a contact occurs between the outside of the displacement element and the inside of the scoop.

For this it is over EP 1 242 743 known to provide internal rotor cooling. With the help of the rotor internal cooling, an effective cooling of the rotor and thus of the at least one displacement element connected to the rotor or integrally formed therewith is ensured, so that small gap heights can be realized. Such an internal rotor cooling is structurally very complex and therefore expensive.

The closest prior art in the form of DE 10 2010 019 402 describes a rotor screw vacuum pump without internal rotor cooling with only one displacement element per rotor, which has a changing pitch.

The object of the invention is to provide a screw vacuum pump with which a high vacuum of in particular less than 200 mbar and particularly preferably less than 10 mbar can be achieved, it being possible to dispense with internal rotor cooling.

The object is achieved according to the invention by a screw vacuum pump according to claim 1.

The screw vacuum pump according to the invention has a housing which forms a scooping space in which the two screw rotors are arranged. According to the invention, the housing and the rotors are made of aluminum or an aluminum alloy. Particularly preferred is the aluminum alloy for the housing AISi7Mg or AlMg0.75Si. In particular, the coefficient of expansion of the material of the screw rotors is less than the coefficient of expansion of the material of the housing. It is particularly preferred that the expansion coefficient of the screw rotors is less than 22 * 10 ^-6 1 / K, particularly preferably less than 20 * 10 ^-6 1 / K.

The two screw rotors arranged in the scoop have at least one displacement element which has a helical recess. The helical recesses form several turns. According to the invention, this is at least one displacement element made of aluminum or an aluminum alloy. It is preferred to produce at least one displacement element made of AISi9Mg or AlSi17Cu4Mg. It is particularly preferred that the aluminum or the aluminum alloy has a low expansion coefficient of in particular less than 22 * 10 ^-6 1 / K, in particular less than 20 * 10 ^-6 1 / K.

It is particularly preferred that the screw rotors and in particular the at least one displacement element per screw rotor have a lower expansion coefficient than the housing. It is particularly preferred here that the expansion coefficient of the housing is at least 5%, particularly preferably at least 10% larger than that of the screw rotors or of the at least one displacement element. It is particularly preferred that the alloy of the rotor has a high silicon content of preferably at least 9%, particularly preferably of more than 15%, in order to realize a low coefficient of thermal expansion.

According to the invention, the screw rotors and the at least one displacement element provided are designed such that at least 6, in particular at least 8 and particularly preferably at least 10 windings are provided between an area in which 5% to 20% of the outlet pressure prevails and the pressure-side rotor end. The rotor end on the pressure side is the area of the pump outlet. The high number of turns in this area according to the invention can be provided in a preferred embodiment in the case of a single pressure-side displacement element provided per rotor. However, it is also possible to provide a corresponding number of turns in this area on the pressure side, for example on two displacement elements. By providing a high number of turns according to the invention in a region in which according to the invention there is then only a relatively low compression of the medium to be conveyed per turn, it is possible to dispense with internal rotor cooling. This is due in particular to the fact that, due to the relatively low compression in this area, the temperature increase caused by the compression of the displacement element is lower. Furthermore, due to the relatively high density of the medium in this area, the conveyed medium itself also provides good heat dissipation from the displacement element into the pump housing.

In addition, the large number of turns means that a large surface is available for heat exchange with the housing.

According to the invention, the at least 6, in particular at least 8 and particularly preferably at least 10 windings are provided in a pressure-side displacement element. It is particularly preferred here that the pressure ratio caused by the pressure-side displacement element (= outlet pressure / intermediate pressure in front of the pressure-side displacement element) is less than 20, in particular less than 10, and particularly preferably less than 5. When compressing to atmospheric pressure at the pump outlet, the last 6, in particular last 8 and particularly preferably last 10 windings in accordance with the invention result in a compression of 50 mbar to 1,000 mbar at a pressure ratio of 20. In this respect, a compression of 100 mbar occurs at a pressure ratio of 10 1,000 mbar and at a pressure ratio of 5 a compression from 200 mbar to 1,000 mbar.

The distance between an area in which 5% - 20% of the outlet pressure prevails, up to the last turn in the conveying direction, i.e. essentially up to the pump outlet is preferably at least 20% to 30% of the rotor length. This in turn has the advantage that in a relatively large area there is only a relatively low compression. This in turn causes a relatively small increase in temperature due to the low compression.

Furthermore, for the inventive design of screw rotors without internal cooling, it is preferred that the pressure-side displacement element has at least 6, in particular at least 8 and particularly preferably at least 10 turns has an average working pressure of more than 50 mbar. When the pump is operating at end pressure, ie when the inlet is closed, a (time-averaged) pressure of 50 mbar is reached at this point of the pump.

According to the invention, it is therefore possible, even in the case of rotors without internal rotor cooling and in a housing made of aluminum or an aluminum alloy and at least one displacement element made of aluminum or aluminum alloy, to have a cold gap between the surface of the at least one displacement element and the inside of the scooping space, in particular in the pressure-side area a height of 0.05 mm - 0.3 mm and in particular 0.1 mm - 0.2 mm. Such a relatively large gap height can be provided on the basis of the configuration according to the invention described above, in particular 6, preferably 8 and particularly preferably 10 last windings.

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 thus has a constant contour and preferably a constant slope. This considerably simplifies the production, so that the production costs can be greatly reduced.

In order to further improve the suction power, the contour of the displacement element on the suction side, that is to say in particular of the first displacement element in the pumping direction, is preferably 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 blowholes, in particular completely disappear or at least have a small cross section. A particularly suitable asymmetrical profile is the so-called "Quimby profile". Such a profile is relatively difficult to manufacture, has the advantage, however, that there is no continuous blow hole. A short circuit only exists between two neighboring chambers. Since it is an asymmetrical profile with different profile flanks, at least two work steps are required for the 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 preferably provided with a symmetrical contour. The symmetrical contour has the particular advantage that production is easier. In particular, both flanks with a symmetrical contour can be produced in one work step by a rotating end mill or by a rotating side milling cutter. Although such symmetrical profiles have blowholes, they are continuous, i.e. not only provided between two adjacent chambers. The size of the blow hole decreases as the slope decreases. In this respect, such symmetrical profiles can be provided in particular in the case of the pressure-side displacement element, since in a preferred embodiment this has a smaller slope than the suction-side displacement element and preferably also as the displacement element arranged between the suction-side and the pressure-side displacement element. Although the tightness of such symmetrical profiles is somewhat lower, they have the advantage that the manufacture is significantly easier. In particular, it is possible to produce the symmetrical profile in a single work step and preferably with a simple end mill or side milling cutter. This significantly reduces costs. 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 constant pitch and constant Contour in a vacuum pump leads to essentially the same results as with a vacuum pump with a displacement element with changing pitch. With high built-in volume ratios, three or four displacement elements can be provided for each 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, that is to say in particular the last, displacement element has a large number of turns. Due to the large number of turns, 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 0.05-0.3 mm. A large number of outlet windings or number of windings in the pressure-side displacement element can be produced inexpensively, since, according to the invention, this displacement element has a constant pitch and preferably 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 6, in particular more than 8 and particularly preferably more than 10 turns. In a particularly preferred embodiment, the use of symmetrical profiles has the advantage that both flanks of the profile can be cut simultaneously with one milling cutter. Here, the milling cutter is additionally supported by the opposite flank, so that deformation or bending of the milling cutter during the milling process and inaccuracies caused thereby are avoided.

To further reduce the 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 erratic. If necessary, the two displacement elements are in the longitudinal direction arranged at a distance from each other 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 led out in a simple manner 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 of such a ring-cylindrical region, is not necessary.

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

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

In the case of displacement elements produced independently of the rotor shaft, these are mounted on the shaft by press fits, for example. Here, it is preferred to provide elements such as dowel pins for fixing the angular position of the displacement elements to one another.

In particular in the one-piece configuration of the screw rotor, but also in the case of a multi-piece configuration, it is preferred to produce it from aluminum or from an aluminum alloy. Particularly preferred is to manufacture the rotor from aluminum or an aluminum alloy, in particular AlSi9Mg or AlMg0.7Si. The alloy preferably has a high silicon content of preferably more than 9%, in particular more than 15%, in order to reduce the coefficient of expansion.

In a further preferred development of the invention, the aluminum used for the rotors has a low coefficient of expansion. It is preferred if the material has an expansion coefficient of less than 22 * 10 ^-6 1 / K, in particular less than 20 * 10 ^-6 1 / K. In a further preferred embodiment, the surface of the displacement elements is coated, a coating against wear and / or corrosion being provided in particular. It is preferred to provide an anodic or other suitable coating depending on the area of application.

It is particularly preferred that the screw rotor is made in one piece, in particular from aluminum or an aluminum alloy. The screw rotor can also have a rotor shaft which carries the at least one displacement element. This has the advantage, in particular when providing a plurality of displacement elements, that these can be produced independently of one another and are subsequently connected to the rotor shaft in particular by being pressed on or shrunk on. It is possible to provide feather keys or the like to define the angular position of the individual displacement elements. The rotor shaft can be made of steel and carry the at least one displacement element made of aluminum or an aluminum alloy.

In the preferred provision of a plurality of displacement elements per screw rotor, it is possible to form the displacement elements in one piece.

It is preferred according to the invention that the screw rotors have no internal rotor cooling. In this respect, it is particularly preferred that the screw rotors have no channels through which liquid coolant flows. However, the screw rotors can have bores or channels, for example for reducing weight, for balancing or the like. It is particularly preferred that the screw rotors are solid.

Furthermore, it is preferred that in the area of the pressure-side displacement elements, i.e. in particular in the area of the last 6, preferably last 8 and particularly preferably last 10 windings, there is a small temperature difference between the displacement elements and the housing. In normal operation, this temperature difference is preferably less than 50 K and in particular less than 20 K. Normal operation means the entire intake pressure range from the final pressure to an open inlet (atmospheric intake).

Furthermore, it is preferred that the housing in the area of the pressure-side displacement elements, ie in particular in the area of the last 6, in particular last 8 and particularly preferably last 10 turns, has an average heat flow density which is less than 20,000 W / m ² , preferably less than 15,000 W / m ² and in particular less than 10,000 W / m ² . The average heat flow density is the ratio of the compression performance to the wall area of the outlet area.

The invention is explained in more detail below on the basis of a preferred embodiment with reference to the attached drawings.

Fig. 1

eine schematische Draufsicht einer ersten bevorzugten Ausführungsform eines Schraubenrotors der erfindungsgemäßen Schraubenvakuumpumpe,

Fig. 2

eine schematische Draufsicht einer zweiten bevorzugten Ausführungsform eines Schraubenrotors der erfindungsgemäßen Schraubenvakuumpumpe,

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 Schraubenvakuumpumpe.

Show it:

Fig. 1

2 shows a schematic top view of a first preferred embodiment of a screw rotor of the screw vacuum pump according to the invention,

Fig. 2

1 shows a schematic top view of a second preferred embodiment of a screw rotor of the screw vacuum pump according to the invention,

Fig. 3

1 shows a schematic sectional view of displacement elements with an asymmetrical profile,

Fig. 4

is a schematic sectional view of displacement elements with a symmetrical profile, and

Fig. 5

is a schematic sectional view of a screw vacuum pump.

The in the 1 and 2 Screw rotors shown can in a screw vacuum pump according to the invention, as in Fig. 5 shown, used.

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

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

At the in Fig. 2 Second preferred embodiment shown are the two displacement elements 10, 12 manufactured separately and then fixed on a rotor shaft 20, for example by pressing. Although this production is somewhat more complex, the cylindrical distance 14 between two adjacent displacement elements 10, 12 is not required as a tool outlet. The bearing seats 16 and the shaft ends 18 can be an integral part of the shaft 20. The continuous shaft 20 can also be made of another material that differs from the displacement elements 10, 12.

Fig. 3 shows a schematic sectional view of an asymmetrical profile (eg a Quimby profile). The asymmetrical profile shown is a so-called "Quimby profile". The sectional view shows two screw rotors which mesh with one another and whose longitudinal direction is perpendicular to the plane of the drawing. The opposite 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 mutually opposite flanks 19, 21 must therefore be produced independently of one another. The production, which is therefore somewhat more complex and difficult, has the advantage, however, that there is no continuous blow hole, but only 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 again shows a cross section of two displacement elements or two screw rotors, which in turn rotate in opposite directions (arrows 15). Relative to the axis of symmetry 17, the flanks 23 are formed symmetrically for each displacement element. At the in Fig. 4 The illustrated preferred embodiment of a symmetrically designed contour is a cycloid profile.

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

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

In the in Fig. 5 Shown schematic view of a preferred embodiment of a screw vacuum pump according to the invention are two screw rotors, as in Fig. 1 shown, arranged in a housing 26. The vacuum pump housing 26 has an inlet 28 through which gas is drawn in in the direction of an arrow 30. The inlet 28 is connected, for example, to a chamber to be evacuated. Furthermore, the pump housing 26 has an outlet 32 on the pressure side, through which the gas is expelled in the direction of an arrow 38. The screw vacuum pump according to the invention preferably pumps directly against the atmosphere, so that no forevacuum pump is connected to the outlet 32, although this is also possible.

In the exemplary embodiment shown, the two pressure-side displacement elements 12 have 10 turns per screw rotor. In particular, there is an area 40, i.e. in a region of the first turn of the pressure-side displacement element 12 in the conveying direction, a pressure of 5% - 20% of the pressure prevailing at the outlet 32.

A gap is formed between the surfaces 42 of the two pressure-side displacement elements 12 and an inner surface 44 of a scooping space 46 formed by the pump housing 26, the height of which is preferably in the range from 0.05 mm to 0.3 mm and in particular in the range from 0.1 mm - 0.2 mm.

The vacuum pump housing 26 is closed in the illustrated embodiment with two housing covers 47. The in Fig. 4 left housing cover 47 has two bearing receptacles, in each of which a ball bearing 48 is arranged for mounting the two rotor shafts. On the in Fig. 4 on the right side, the pins 50 of the two screw rotor shafts protrude through the covers 47. On the outside, a gear wheel 52 is arranged on each of the two shaft journals 50. In the exemplary embodiment shown, the two gear wheels 52 mesh with one another in order to synchronize the two screw rotors with one another. Furthermore, in the in Fig. 4 right cover 47 two bearings 48 arranged for storing the screw rotors.

At the in Fig. 5 lower shaft is the drive shaft, which is connected to a drive motor, not shown.

Particularly good results according to the invention could be achieved with the following configuration, which is therefore particularly preferred: Housing material AlSi7Mg (cast, expansion coefficient 22 ^∗ 10 ^-6 K ^-1 or AlMg0.7Si (extrusion, expansion coefficient 23 ^∗ 10 ^-6 K ^-1 ) Material rotor AlSi9Mg (cast iron, coefficient of expansion 21 ^∗ 10 ^-6 K ^-1 ) or AlSi17Cu4Mg (cast iron, coefficient of expansion 18 ^∗ 10 ^-6 K ^-1 ) Silicon content rotor at least 9%, particularly preferably more than 15% Coefficient of thermal expansion Housing / rotor at least 5% larger, particularly preferably 10% larger Intermediate pressure between the suction-side and the pressure-side displacement element:

Pressure ratio Outlet pressure / intermediate pressure

Zwischendruck = 20% Auslassdruck Particularly preferably less than: $$\frac{1000\mathit{mbar}}{200\mathit{mbar}}=5$$

Intermediate pressure = 20% outlet pressure

Zwischendruck = 10% Auslassdruck weniger als $$\frac{1000\mathit{mbar}}{50\mathit{mbar}}=20$$

Zwischendruck = 5% Auslassdruck Kaltspalt Höhe 0,05 mm - 0,3 mm Besonders bevorzugt 0,1 mm - 0,2 mm In particular less than $$\frac{1000\mathit{mbar}}{100\mathit{mbar}}=10$$

Intermediate pressure = 10% outlet pressure less than $$\frac{1000\mathit{mbar}}{50\mathit{mbar}}=20$$

Intermediate pressure = 5% outlet pressure Cold gap height 0.05 mm - 0.3 mm Particularly preferably 0.1 mm - 0.2 mm

Claims (15)

1. A screw-type vacuum pump, comprising
   a housing (26) defining a suction chamber, wherein the housing (26) is made of aluminum or an aluminum alloy,
   two screw rotors arranged in said suction chamber (46), each comprising at least one displacer element (10, 12) having a helical recess for forming several windings, wherein the at least one displacer element (10, 12) is made of aluminum or an aluminum alloy,
   characterized in that
   in case of a prevailing suction pressure of less than 200 mbar (absolute pressure) between the region in which 5% to 20% of the outlet pressure prevail and a pressure-side rotor end (pump outlet) at least six, particularly at least eight and with particular preference at least ten windings are provided, and
   that the one displacer element is formed as a pressure-side displacer element (12) and at least one further displacer element (10) is provided per screw rotor, wherein the recesses of the displacer elements (10, 12) each have the same contour along the entire length.
2. The screw-type vacuum pump of claim 1, characterized in that the pressure-side displacer element (12) effects a pressure ratio of less than 20, particularly less than 10 and with particular preference less than 5.
3. The screw-type vacuum pump of claim 1 or 2, characterized in that the pressure-side displacer element (12) at a minimum of 6, particularly at a minimum of 8 and with particular preference at a minimum of 10 windings has an average working pressure of more than 50 mbar.
4. The screw-type vacuum pump of claim 1 to 3, characterized in that between a surface (42) of the displacer element (12) and an inner surface (44) of the suction chamber (46) a gap is formed having a height in the range from 0.05 mm to 0.3 mm, particularly from 0.05 mm to 0.2 mm.
5. The screw-type vacuum pump of one of claim 1 to 4, characterized in that the pressure-side displacer elements (12) comprise a constant pitch and/or a uniform symmetric contour along their entire length.
6. The screw-type vacuum pump of one of claim 1 to 5, characterized in that the recesses of the pressure-side displacer elements (12) comprise a uniform symmetric contour along their entire length.
7. The screw-type vacuum pump of one of claim 1 to 6, characterized in that each screw rotor comprises a rotor shaft carrying the at least two displacer elements (10, 12).
8. The screw-type vacuum pump of one of claim 1 to 7, characterized in that a screw rotor made of aluminum or an aluminum alloy has a low expansion coefficient of and particularly of less than 22*10^-61/K and with particular preference of less than 20*10^-61/K.
9. The screw-type vacuum pump of one of claim 1 to 8, characterized in that the screw rotor and particularly the at least one displacer element (10, 12) comprise a lower expansion coefficient per screw rotor than the housing (26), wherein the expansion coefficient of the housing (26) is particularly at least larger than the expansion coefficient of the screw rotors and of the at least one displacer element (10, 12), respectively.
10. The screw-type vacuum pump of one of claim 1 to 9, characterized in that the screw rotors do not comprise a rotor internal cooling.
11. The screw-type vacuum pump of one of claim 1 to 10, characterized in that the screw rotors do not comprise channels with particularly liquid coolant flowing through them.
12. The screw-type vacuum pump of one of claim 1 to 11, characterized in that the screw rotors are configured to be solid.
13. The screw-type vacuum pump of one of claim 1 to 12, characterized in that in normal operation a temperature difference in the region of the pressure-side displacer elements (12) between said displacer elements and the housing (26) is smaller than 50 K and particularly smaller than 20 K.
14. The screw-type vacuum pump of one of claim 1 to 13, characterized in that in the region of the pressure-side displacer elements (12) the average heat flux density is less than 20000 W/m², preferably less than 15000 W/m² and particularly less than 10000 W/m².
15. The screw-type vacuum pump of one of claim 1 to 14, characterized in that the distance from the region in which 5% to 20% of the outlet pressure prevail to the last winding of the pressure-side displacer element (12) is at least 20% to 30% of the rotor length.

Images