EP3499041B1 - Screw vacuum pump - Google Patents

Description

The present invention relates to a screw vacuum pump having a housing, two screw rotors which are arranged in the housing and are in engagement with one another and which, in order to convey a process gas in cooperation with the housing, form repeatedly closed delivery volumes of the process gas and convey them in the direction of an outlet.

Screw vacuum pumps generally include complex pump-active geometries, in particular screw profiles of screw rotors and a corresponding housing for the screw rotors. The interaction of these geometries requires high precision in their manufacture. It can therefore be very complex and expensive to manufacture or control the geometries.

A screw vacuum pump according to the preamble of claim 1 is in the DE 20 2016 005 209 U1 disclosed. The DE 20 2016 005 208 U1 , the US 2002/031439 A1 , the DE 100 19 637 A1 and the JP 4 900270 B2 disclose further screw vacuum pumps with screw rotors, which have different sections.

It is an object of the invention to reduce the required manufacturing precision at least for a sub-area of a pump-active geometry.

This object is achieved by a screw vacuum pump with the features of claim 1, and in particular by the fact that the screw rotors each have at least two sections adjacent along the screw axis, the screw rotors each having an at least substantially constant and in a first section closer to an inlet in a second section have a lower slope than in the first section, and wherein with respect to the screw axis, the first section is longer than a completed delivery volume in the first section.

At an inlet of the screw vacuum pump, a respective screw profile interacts with the housing to include a delivery volume. Provided If the delivery volume to be closed reaches the second section opposite to the inlet, this means that the process gas is partially expelled back towards the inlet. For this reason, high manufacturing precision of the housing in the inlet area was previously necessary to ensure a precisely defined completion of the delivery volume. Precise manufacture of the inlet area is complex and is made more difficult, for example, by the fact that a mold for the housing wears out over several casting cycles.

Since the first section is longer than a completed delivery volume according to the invention, it is ensured that the second section is not reached before the delivery volume is completed. The manufacturing precision of the housing in the inlet area can thus be reduced without running the risk of the process gas knocking back towards the inlet.

The reduced requirement for manufacturing precision considerably simplifies the manufacture of the vacuum pump. It allows a more stable manufacturing process with fewer defective components. This is particularly advantageous because the housing in the inlet area is often difficult to measure, in particular only with great effort using a 3D coordinate measuring system. Without a measurement, deviations may only be recognized during a final inspection of the screw vacuum pump, which in the event of a fault leads to high costs for reassembly. The influence of manufacturing precision is reduced by the invention, so that deviations from a desired shape are less relevant or harmless.

In addition, the invention not only simplifies the manufacture of the housing, but also the manufacture of the screw rotors, since larger tools and / or tools with a higher chip volume can be used to produce the profile in the relatively long first section with a relatively large pitch. This reduces the processing time and the associated manufacturing costs. This can further reduce the processing time, for example be that a respective screw contour of the screw rotors, in particular only or also in the first section, is already formed during casting at least in a preliminary contour for further machining.

It has been shown that, despite the lengthened first section, efficient screw profiles can be designed with a comparable overall length of the rotors. The manufacturing advantages outweigh the vacuum restrictions, so that the invention makes it possible to manufacture an efficient pump in a particularly simple manner.

It has also surprisingly been found that the screw vacuum pump according to the invention even has an improved pumping speed at relatively high suction pressures. This is attributed to the fact that, with the longer first section, the gaps in the first section which form between the rotors with one another and in particular with respect to the housing are longer. The process gas in the area of the internal compression, which results in particularly high internal pressures at high intake pressures, must therefore overcome longer gaps in order to flow back. The backflow is consequently made more difficult or better sealed. The screw vacuum pump according to the invention is therefore not only particularly easy to manufacture with justifiable design restrictions, but also has improved vacuum performance in certain pressure ranges.

A closed delivery volume can generally be understood to be a closed area - apart from unavoidable gaps between the rotors and in relation to the housing - which extends between two screw threads of a respective rotor along the screw profile around the rotor and outwards from the housing and along the screw profile from each other rotor is limited. The delivery volume moves during the pumping process along the respective screw rotor in its profile from the inlet to the Outlet, providing the pumping action. The funding volume can decrease on the way between inlet and outlet. This is called internal compression.

In particular, the closed delivery volume can cover an angle of 360 ° around the screw axis of the respective rotor.

In one embodiment, the first section corresponds to at least 1.25 times, in particular at least 1.5 times, in particular at least 1.75 times, in particular at least 2 times the length of the delivery volume. The advantages mentioned above are thereby increased accordingly.

An end face for the screw rotors is formed in a suction area of the pump, the shape of which is adapted to the screw rotors in such a way that a respective delivery volume can be opened for suction and can be closed for delivery.

The end face in particular runs at least substantially perpendicular to the screw axes. The end surface forms e.g. one, in particular inner, end face of the housing. The end surface can preferably be designed as a free-form surface and / or run at least substantially parallel to at least one corresponding screw profile. In particular, two areas of the end face that are parallel to the screw profiles can be provided, which are brought together in particular to form an area that is raised, in particular lower, and / or lowered, in particular upper, with respect to the screw axis.

The end surface can generally be formed, for example, in the housing. The housing can be designed as a cast part, for example. In one embodiment, the end surface is in a casting, in particular the housing, trained and unprocessed. This completely saves the manufacturing step of machining the end face, which further simplifies the manufacture of the housing.

In a further embodiment, a screw profile of a respective screw rotor is formed by a cycloid. Such a screw profile can advantageously be designed as required.

In one development, a screw profile of a respective screw rotor is designed with two threads. This results in a particularly low imbalance of the corresponding screw rotor. Thus, compensation elements such as Compensating masses that require additional installation space and / or compensating holes in which material can be deposited can be dispensed with. On the other hand, higher speeds can be made possible.

In a further embodiment, the screw rotors each have an at least substantially constant pitch in the second section. Such screw rotors are particularly easy to design and manufacture because the constant pitch forms a relatively simple geometry.

It can be provided that the screw rotors each have at least a third section and their pitch is smaller in the third section than in the second section. This allows additional internal compression to be achieved.

The slope in the third section can in particular be at least substantially constant. In a further exemplary embodiment, the screw rotors each have a plurality of sections of different pitch over their entire active pumping length, the pitch in all sections, in particular is constant at least in areas or subsections, in particular in each case. Both contribute to a simplified design and manufacture.

Between two adjacent sections of constant pitch, the pitch of the screw profile along a screw axis basically only changes in transition areas between the sections. A transition area is in particular smaller than a section, in particular than the adjacent sections. In particular, all transition areas are smaller than all sections.

The screw vacuum pump can, for example, have an internal compression with a compression ratio of less than 5 to 1, in particular less than 4 to 1, in particular less than 3.5 to 1, and / or greater than 2 to 1, in particular greater than 3 to 1. A respective screw profile can e.g. Form or promote more than 7, in particular more than 10, in particular more than 12, in particular 13, closed delivery volumes at the same time. A respective screw rotor can in particular have a ratio of the length of its screw profile to its diameter of at least 2.0, in particular at least 2.5, in particular at least 3.0 and / or at most 5.0, in particular 4.0.

Fig. 1

zeigt eine Schraubenvakuumpumpe in perspektivischer Ansicht.

Fig. 2

zeigt die Schraubenvakuumpumpe der Fig. 1 in einer Draufsicht.

Fig. 3

zeigt die Schraubenvakuumpumpe der Fig. 1 und 2 in einer Seitenansicht.

Fig. 4

zeigt eine Schnittansicht der Schraubenvakuumpumpe entlang einer in Fig. 3 angedeuteten Schnittebene A-A.

Fig. 5

zeigt einen Tauchkühler der Schraubenvakuumpumpe der Fig. 1 bis 4.

Fig. 6

zeigt einen Ansaugbereich der Schraubenvakuumpumpe der Fig. 1 bis 4 in geschnittener, perspektivischer Ansicht.

Fig. 7

zeigt die Schnittdarstellung der Fig, 6 in einer Seitenansicht.

Fig. 8

zeigt ein Gehäuse der Schraubenvakuumpumpe in perspektivischer Ansicht.

Fig. 9

zeigt in einem Diagramm zwei Saugvermögensverläufe jeweils von einer Schraubenpumpe mit einer bearbeiteten bzw. unbearbeiteten Abschlussfläche.

The invention is explained below by way of example only with reference to the schematic drawing.

Fig. 1

shows a screw vacuum pump in perspective view.

Fig. 2

shows the screw vacuum pump of the Fig. 1 in a top view.

Fig. 3

shows the screw vacuum pump of the Fig. 1 and 2nd in a side view.

Fig. 4

shows a sectional view of the screw vacuum pump along an in Fig. 3 indicated cutting plane AA.

Fig. 5

shows an immersion cooler of the screw vacuum pump 1 to 4 .

Fig. 6

shows a suction area of the screw vacuum pump of the 1 to 4 in a cut, perspective view.

Fig. 7

shows the sectional view of the Fig. 6 in a side view.

Fig. 8

shows a housing of the screw vacuum pump in a perspective view.

Fig. 9

shows in a diagram two pumping speed curves, each from a screw pump with a machined or unprocessed end surface.

In the 1 to 3 A screw vacuum pump 10 is shown, which has a motor 12, a gear box 14, a housing 16, a bearing plate 18 and a cover 20. The screw vacuum pump 10 conveys a process gas from an inlet 22 to a downward, in Fig. 3 visible outlet 24.

An active liquid cooling is provided for the motor 12, which emerges from a housing of the motor 12. For arranged inside the housing 16 and in Fig. 4 visible screw rotors 28 and 30 an active liquid cooling is also provided, which has two cooling lines, which in Fig. 1 are not shown, but the course thereof by means of corresponding grooves 32 in the housing 16 is indicated, into which the cooling lines are pressed. Furthermore, active liquid cooling systems are provided in the gearbox 14 and in the cover 20 and are each designed here as immersion coolers 34, which are described below with reference to FIG Fig. 5 are explained in more detail.

Like it in the 1 to 4 can be seen, the housing 16 of the screw vacuum pump 10 has a waist 36. The waist 36 is arranged in the region of the outlet 24.

In Fig. 4 the screw vacuum pump 10 is shown in a sectional view, the sectional plane of the line AA in Fig. 3 corresponds. Two screw rotors 28 and 30 are visible, each having two-start, interlocking screw profiles 38 and 40, which are generated with the aid of a cycloid profile and have a cylindrical envelope contour and a cylindrical basic shape of the screw base. In cooperation with the housing 16, the screw profiles 38 and 40 form a pump-active area of the screw vacuum pump 10 and repeatedly convey closed delivery volumes of the process gas from the inlet 22 to the outlet 24, in Fig. 4 from left to right.

The pumping power of the screw vacuum pump 10 depends on the size and shape of various gaps in the pumping area, which are unavoidable due to the relative movement of the rotors 28, 30 and housing 16, but are small and as constant as possible for good pumping power. Temperature changes in the components involved lead to their shape change. The measures described here for avoiding, dissipating and, in general, controlling heat in the pump 10 thus bring about the least possible change in shape and consequently the most manageable gaps. The gaps can therefore be designed more precisely, which improves the pump performance and its efficiency.

The screw rotor 28 is driven directly by the motor 12, that is to say without an intermediate coupling. In contrast, the screw rotor 30 is driven via a synchronization gear 42 with gearwheels 43 in a defined angular relationship to the screw rotor 28.

The motor 12 comprises a housing 44, which is made of aluminum, for example, and in which cooling lines 26 are formed for the active liquid cooling. The motor 12 also includes a wound stator 46 which, together with a magnet carrier 48 attached to a shaft end of the screw rotor 28, forms an electric motor and a direct drive for the screw rotor 28. The screw rotor 28 forms a rotor of the motor 12. The magnet carrier 48 comprises a plurality of permanent magnets. The motor 12 thus forms a permanent magnet synchronous machine with internal magnets, which is also referred to as IPMSM.

The stator 46 is arranged in a potting body 50, which insulates electrical conductors (not shown in more detail) in the stator 46 and insulates them to a circuit board 52. The potting body 50 here, in conjunction with the circuit board 52, forms a vacuum-tight connection of the motor 12 to control electronics provided in a range of atmospheric pressure. For example, an external frequency converter can be provided for the motor 12. Alternatively or additionally, at least part of control electronics for the motor 12 can be provided on the circuit board 52.

The synchronizing gear 42 is arranged in the gear box 14. In the gear box 14, oil is also provided as a lubricant, which is distributed by spray disks 54 via the synchronizing gear 42 and adjacent bearings 56.

The waist 36 forms a shield or a heat barrier, in particular for heat that is produced in the area of the screw rotors 28, 30 during pump operation. Due to the fact that a small material cross section remains, and because the change in shape extends a heat path, the heat from the screw rotor, which otherwise spreads in the housing 16, is prevented from reaching areas beyond. In particular, the oil in the gearbox 14 and the bearings 56 are protected from excessive temperatures. The immersion cooler 34 arranged in the gearbox 14 also contributes to reducing the temperature. This is arranged in an oil bath (not shown) of the gear box 14 and thus cools the oil directly.

A lubricant discharge device designed as a deflector 58 is provided for a respective screw rotor 28 or 30 adjacent to the bearings 56, which form a fixed bearing here. A respective deflector 58 forms a barrier for the oil in the gearbox so that it does not get into a pump-active area or a vacuum area, here in particular an outlet area. The deflector 58 comprises a not shown edge for the oil. Opposite the fling edge, a drain groove is formed in the housing 16, which takes up flung oil and directs it back into the gear box 14 or into an oil bath there. The oil, which is conveyed or distributed by the spray disks 54 to the gear 42 and bearing 56, is thus removed again by the deflectors 58 from the rotors 28 and 30, respectively.

Piston rings are provided on a piston ring carrier 60 as a dynamic fluid seal. These form a non-contact seal and thus avoid frictional heat. The deflectors 58 return as much oil as possible to the gearbox 14, so that as little oil as possible is present on the piston rings. An overall reliable sealing effect is achieved with particularly low heat production.

The screw rotors 28 and 30 have three sections of different pitch in their respective screw profile 38 and 40, respectively. A first section 62 in the pumping direction, in Fig. 4 left, forms a suction section and has a constant and the largest slope among the three sections. The first section 62 is longer than a closed delivery volume in the first section with respect to a screw axis 63 which runs along a respective rotor 28 or 30. A second section 64 has a plurality of subsections, which are not referenced in any more detail, with different but in each case constant slopes in the screw profile 38 or 40, the slopes being lower than in the first section. The second section 64 forms the longest section here. A third section 66 with an even lower slope forms an ejection section. In the third section there is again a constant slope. Due to the reduced slope along the pump direction, an internal compression is brought about, which compresses the process gas before it is expelled.

The rotors 28, 30 and the screw profiles 38, 40 can be designed and manufactured particularly simply by providing the constant sections. As it is based on Fig. 4 It can be seen that an elongated first section 62 leads to correspondingly elongated gaps between the screw profiles 28, 30 and the housing 16, so that the path or the column from the internal compression at the transition of the sections 62 and 64 to a scooping area or suction area 67 is longer is. The sealing effect of the gaps is correspondingly increased, which leads to improved sealing of the internal compression with respect to the suction area 67, in particular at high differential pressures.

The screw vacuum pump 10 thus has an internal compression. In cooperation with the housing 16, the screw rotors 28, 30 of the pump 10 include repeatedly closed delivery volumes. Their size is larger at an inlet end or in section 62 than at an outlet end End or in section 62. The size of a delivery volume is determined by a cross section of a screw profile 38, 40 and its slope.

The size of a delivery volume on the inlet side or in section 62 determines a theoretical pumping speed of the screw pump 10. The slope of the screw profile 38, 40 is constant on the inlet side via section 62, so that the delivery volume is only compressed after completion by the internal compression. If a respective rotor 28, 30 closes a respective delivery volume too early or too late or if the internal compression begins too early, the theoretical pumping speed of the pump drops.

The size of a respective delivery volume on the outlet side or in section 66 determines the power consumption of the pump in operation at an achievable final pressure. The ratio of the sizes of the delivery volume on the inlet side and outlet side or in sections 62 and 66 corresponds to the ratio of the internal compression of the pump.

In section 66, the pitch is constant over several revolutions of the screw profile 38, 40. The slope corresponds approximately to the minimum of the slope that can be achieved by a specific machining tool and is therefore, particularly in consideration of costs, production-related. Because several revolutions, that is to say a plurality of closed delivery volumes, are provided in section 66, a backflow due to a pressure difference between the gaps is compensated. Overall, in particular the overall gradient course along the rotors 28, 30 and the size of the gaps formed between the rotors 28, 30 and between rotors 28, 30 and the housing 16 determine the vacuum performance data of the pump, in particular the pumping speed and an achievable final pressure.

The screw profiles 38, 40 have a particularly low unbalance due to their two-start design. For example, there are no compensating elements, e.g. Compensating compounds that require additional installation space and / or compensating holes in which material can be deposited are necessary. The pump can be operated with the two-start cycloid screw profiles 38, 40 in a wide speed range, in particular with speed control, and / or, for example, in a stand-by mode.

The compression of the process gas generally generates heat, which in the screw pump 10 is dissipated primarily by liquid cooling. In Fig. 4 the grooves 32 provided for this purpose are visible. Cooling lines of the liquid cooling extend here and preferably in the longitudinal direction over a wide range of the screw profiles, in particular over at least half the length of the screw profiles. In particular, the liquid cooling is arranged in the area or in the vicinity of an internal compression.

The bearing plate 18 is fastened to an inlet-side end of the housing 16. Among other things, this carries a further bearing with bearings 68, which form a loose bearing. In contrast to an opposite bearing plate 70 arranged on an outlet-side housing end, which is formed integrally with the housing 16 but can also be formed separately, the bearing plate 68 is formed as a separate component, but can also be formed integrally.

Spray disks 54, deflectors 58 and a piston ring carrier 60 with a plurality of piston rings are also provided on the inlet side and operate in accordance with the arrangement on the outlet side. On the inlet side, a further oil bath, which is carried out separately, is provided in the cover 20. An immersion cooler 34 is also provided for this oil bath. Alternatively or additionally, for example, a cooling line can also be provided, in particular cast, in a wall of the end shield 14 and / or the cover 20

At the beginning of a pumping process by the pump 10, there is usually essentially the same pressure at the inlet 22 as at the outlet. In contrast, during the pumping down, the pressure at the inlet 22 drops to a final pressure which is essentially zero with regard to the resulting forces. Thus, the pressure at the outlet 24 exerts a force on the rotors 28, 30 that is different than at the beginning of the pumping process. To balance this force e.g. a biasing device, in particular a spring, may be provided, which is provided in particular in the case of a loose bearing of the rotor and / or on the inlet side. The pretensioning device can, for example, also absorb forces acting on the rotors through helically toothed gearwheels and / or generally ensure that the bearings are preloaded in accordance with the design, regardless of the operating state when the pressures or pressure conditions change.

In Fig. 5 An immersion cooler 34 is shown as it is arranged in the gearbox 14 or in the cover 20 of the screw vacuum pump 10. In this embodiment, the immersion coolers 34 are therefore of identical design, which leads to a small number of parts and low manufacturing costs.

The immersion cooler 34 has a cooling line 72 which runs through a heat sink 74. The heat sink has a structure for increasing the surface of the heat sink in order to optimize the heat transfer. The immersion cooler 34 also has a flange 76 with which the immersion cooler 34 is fastened.

In Fig. 6 the suction area 67 of the screw vacuum pump 10 is illustrated in a perspective sectional view. In the suction area 67, respective suction-side ends of the screw profiles 38 and 40 are arranged, which due to their rotation and in cooperation with an end surface 78 repeatedly close delivery volumes and convey them along the rotor axes to the outlet 24.

The end surface 78 is essentially designed as a free-form surface, ie it has a relatively complex shape that deviates from a simple plane. For example, the end surface 78 has a roof-like, lower area 80 and a valley-like or groove-like, upper area 82. Machining the end surface 78 would therefore be very complex, especially if high precision is required. The end face 78, however, is unprocessed. It is formed by casting the housing 16 by an appropriate shape. Although this creates a relatively imprecise end face 78, this is essentially unproblematic because of the first section 62, which is relatively long in accordance with the invention. Machining can be saved here advantageously.

The sectional view of the Fig. 6 is in Fig. 7 shown in a side view to further illustrate the course of the end surface 78. Only the screw rotor 30 is visible, since it hides the screw rotor 28 in this view.

As in Fig. 7 can be seen, the end face 78 runs in a region surrounding the screw rotor 30 essentially parallel or corresponding to its screw profile 40. When the rotor 30 is in relation to one another Fig. 6 rotates counterclockwise, the distance between the screw profile 40 and the end face 78 decreases until the distance is zero and a delivery volume is closed in the corresponding profile space 84.

In Fig. 8 the housing 16 of the screw vacuum pump 10 is shown in perspective view without the screw rotors 28, 30. Here it becomes clear once again that the complex end surface 78 can only be brought into a desired shape with great effort, in particular by machining processes such as ripping. Because of its complex shape, the end surface 78 cannot be measured or can only be measured with great effort and therefore also hides a danger for series production, since any defects can only be uncovered during a final inspection of the pump without extensive measurement. This goes hand in hand with high assembly and manufacturing costs and expensive rejects. The invention generally reduces the influence of the accuracy of the end face 78 on the vacuum performance data.

Fig. 9 shows a diagram, the abscissa of which represents a pressure p in hectopascals of a process gas at the inlet of a screw vacuum pump under investigation and whose ordinate represents a pumping speed S in cubic meters per hour of the pump. Two curves 86 and 88 of the pumping speed as a function of the inlet pressure are shown.

The dashed curve 86 is based on an exemplary screw vacuum pump with an inlet area or with an end surface that has been machined to a desired contour. The length of a first section of constant slope essentially corresponds to the length of a closed delivery volume in the first section. The exemplary screw vacuum pump of the course 86 is therefore not designed according to the invention.

The curve 88 shown as a solid line was recorded for a further exemplary screw vacuum pump, in which an oversize intended for machining the end face was left and was not machined. Otherwise, the screw vacuum pump of the course 88 is identical to the screw vacuum pump of the course 86. The two pumps or end faces only differ by a certain allowance. The oversize is provided in the direction of the rotor axes, which is why with unchanged rotors a respective delivery volume is completed earlier and the first section is therefore longer than a delivery volume. The screw vacuum pump of the course 88 is consequently designed according to the invention.

Both curves 86 and 88 have a pumping speed curve typical of screw vacuum pumps with internal compression. The pumping speed is greatest in a medium pressure range. In contrast, the pumping speed at high pressures or at the beginning of a pumping process is in Fig. 9 right, lower. The pumping speed drops when a final pressure is reached, in Fig. 9 left, to zero.

In the middle area, the course 86 shows a higher pumping speed. It is noticeable here that the delivery volumes are precisely closed due to the end surface being precisely manufactured to the target contour. This effect is indicated by arrow 90. The pumping speed according to curve 86 essentially corresponds to a theoretical pumping speed. A backflow due to compression, in particular to a second section, is therefore not a dominant effect in this middle pressure range.

An arrow 92 indicates that the achievable final pressure in the screw vacuum pump with oversize, that is to say with the course 88, is lower or better. This is attributed to the fact that the overall pump-active length of the screw rotors is greater due to the measurement. The gaps between the rotors and the housing are correspondingly longer, so that overall a better seal against backflows from the outlet to the inlet is achieved.

Surprised by the Fig. 9 underlying experiments showed that the screw vacuum pump with oversize or the course 88 also has an improved pumping speed at high pressures, which is indicated by arrow 94. This is attributed to the fact that there is a gap between an internal compression of the pump and a suction area which is lengthened due to the oversize, which in turn leads to a correspondingly improved sealing against backflow. This is particularly noticeable in the area of high pressures, since here the internal compression leads to the highest internal pressures or differential pressures acting via gaps.

It turns out that the screw vacuum pump according to the invention, in particular its housing, is particularly simple to manufacture, since the influence of the corresponding surface accuracy has been reduced. In particular, this eliminates the need for complex machining of the housing in the inlet area or on an end surface. A negative influence on the vacuum performance is small and in some pressure ranges the invention even leads to an improvement in the performance data.

Reference list

10th

Screw vacuum pump

12th

engine

14

Gear box

16

casing

18th

End shield

20

cover

22

inlet

24th

Outlet

26

Cooling pipe

28

Screw rotor

30th

Screw rotor

32

Groove

34

Immersion cooler

36

Waist

38

Screw profile

40

Screw profile

42

Synchronization gear

43

gear

44

casing

46

stator

48

Magnetic carrier

50

Potting body

52

circuit board

54

Washer

56

camp

58

Deflector

60

Piston ring carrier

62

first section

63

Screw axis

64

second part

66

third section

67

Suction area

68

camp

70

End shield

72

Cooling pipe

74

Heatsink

76

flange

78

Finishing area

80

lower area

82

upper area

84

Profile space

86

Pumping speed curve

88

Pumping speed curve

90

arrow

92

arrow

94

arrow

p

Inlet pressure

S

Pumping speed

Claims (14)

1. A screw vacuum pump (10) comprising
   a housing (16);
   two screw rotors (28, 30) which are arranged in the housing (16), which are in engagement with one another and which, for the conveying of a process gas, repeatedly form closed conveying volumes of the process gas in cooperation with the housing (16) and convey them in the direction of an outlet (24),
   wherein the screw rotors (28, 30) each have at least two sections (62, 64) which are adjacent along the screw axis (63);
   wherein the screw rotors (28, 30) each have an at least substantially constant pitch in a first section (62) disposed closer to an inlet (22) and have a lower pitch in a second section (64) than in the first section (62); and wherein the first section (62) is longer with respect to the screw axis (63) than a closed conveying volume in the first section (62),
   characterized in that
   a termination surface (78) for the screw rotors (28, 30) is formed in a suction region (67) of the pump and is adapted with respect to its shape to the screw rotors (28, 30) such that a respective conveying volume can be opened for suction and closed for conveying.
2. A screw vacuum pump (10) in accordance with claim 1,
   characterized in that
   the closed conveying volume covers an angle of 360° about the screw axis (63) of the respective rotor (28, 30).
3. A screw vacuum pump (10) in accordance with claim 1 or claim 2,
   characterized in that
   the first section (62) corresponds to at least 1.25 times the length of the conveying volume.
4. A screw vacuum pump (10) in accordance with at least one of the preceding claims,
   characterized in that
   the first section (62) corresponds to at least 1.5 times the length of the conveying volume.
5. A screw vacuum pump (10) in accordance with at least one of the preceding claims,
   characterized in that
   the first section (62) corresponds to at least 1.75 times the length of the conveying volume.
6. A screw vacuum pump (10) in accordance with at least one of the preceding claims,
   characterized in that
   the first section (62) corresponds to at least twice the length of the conveying volume.
7. A screw vacuum pump (10) in accordance with at least one of the preceding claims,
   characterized in that
   the termination surface (78) is formed in a casting (16) and is not machined.
8. A screw vacuum pump (10) in accordance with at least one of the preceding claims,
   characterized in that
   the termination surface (78) is formed in the housing (16).
9. A screw vacuum pump (10) in accordance with at least one of the preceding claims,
   characterized in that
   a screw profile (38, 40) of a respective screw rotor (28, 30) is formed by a cycloid.
10. A screw vacuum pump (10) in accordance with at least one of the preceding claims,
    characterized in that
    a screw profile (38, 40) of a respective screw rotor (28, 30) is configured as a two-start screw profile.
11. A screw vacuum pump (10) in accordance with at least one of the preceding claims,
    characterized in that
    the screw rotors (28, 30) each have an at least substantially and at least regionally constant pitch in the second section (64).
12. A screw vacuum pump (10) in accordance with at least one of the preceding claims,
    characterized in that
    the screw rotors (28, 30) each have at least one third section (66) and their pitch is smaller in the third section (66) than in the second section (64).
13. A screw vacuum pump (10) in accordance with claim 12,
    characterized in that
    the pitch is at least substantially constant in the third section (66).
14. A screw vacuum pump (10) in accordance with at least one of the preceding claims,
    characterized in that
    the screw rotors (28, 30) each have a plurality of sections (62, 64, 66) of different pitches over their total pump-active length, with the pitch being constant in all the sections (62, 64, 66).

Images