Title of the invention: Vacuum pump

Technical field of the invention

The invention concerns the field of vacuum pumps and, more precisely, a vacuum pump enabling a primary vacuum to be obtained.

Technical background

Some vacuum pumps include pumping stages mounted in series, each including a pair of rotors turning in a synchronized manner and in opposite directions in a closed volume delimited by a stator in order to pump a fluid between a suction inlet and a discharge outlet.

In a multistage vacuum pump the compression volume between the rotors and the stator decreases along the successive stages to increase the pressure on approaching the discharge outlet. In fact, along the pumping stages, the pressure increases to a required discharge pressure generally higher than the ambient pressure in which the vacuum pump is installed, typically an atmospheric pressure of around 1 bar (10 ⁵ Pa).

The power P consumed by a vacuum pump depends on the torque C exerted on the rotors and on their rotation speed w. The torque is equal to the sum of the torques 6C exerted on the rotors of each stage. Each torque is a function of the pressure difference Dr between the upstream side and the downstream side of the pumping stage, of the projected section of the rotors and of the thickness e of the pumping stage. The pressure difference Dr of the final pumping stages being the greatest, the final pumping stages, that is to say the stages closest to the discharge outlet, are those that require the most electrical power.

In the vacuum pumps field it is usual to increase the number of pumping stages to reduce the pressure differences Dr between the stages or to add an auxiliary vacuum pump in series with the main pumping flow to reduce the maximum inlet pressure in the final pumping stages.

Another solution for reducing the electrical power consists in reducing the thickness of the final stages. However, the mechanical production limits are now being reached where the thinness of the stages is concerned, in particular because of the rigidity properties of the materials.

Summary of the invention

A particular object of the invention is to provide a new twin shaft vacuum pump architecture enabling reduction of its electrical power. To this end, the invention has for object a vacuum pump including at least two pumping stages mounted in series, each pumping stage being formed by a compression chamber delimited by a stator of the vacuum pump and receiving a pair of rotors each configured to be driven by a shaft about a respective rotation axis and having conjugate pumping profiles in order to pump a gas in a circulation direction extending from a suction inlet to a discharge outlet, characterized in that at least each rotor of the pair of the final pumping stage upstream of the discharge outlet comprises a support disk made in one piece with the pumping profile, the compression chamber of the final pumping stage featuring two cylindrical cavities complementary to the pair of rotors, the pumping profile of each rotor of the pair of the final stage being configured to turn in a first part of the cylindrical cavity and the support disk being configured to turn in a second part of the cylindrical cavity, the first and second parts of the cylindrical cavities being axially inverted one relative to the other, the first parts overlapping transversely so that the pumping profiles of the pair are able to cooperate with one another to pump a gas.

With a (monobloc) pumping profile produced in one piece with a support disk it is possible to produce much thinner pumping profiles, the support disks providing the mechanical strength of the entire rotor. The thicknesses of the pumping profiles can then be reduced and these thin profiles can be produced accurately, at least in the final pumping stage, which makes it possible to be able to reduce the electrical power of the vacuum pump. The cavities of the compression chamber of the final pumping stage are also enlarged to receive the support disks. They are thus easier to machine.

Another advantage is that this arrangement enables increased conductances at the shaft passages of the final pumping stage, which enables reduction of gas leaks through the shaft passages, between the shafts and the orifices in the transverse walls of the compression chambers. In fact, the support disks of the rotors force the leaking gases to circumvent the edges of the disks rather than to flow along the shafts. It is then more difficult for the gases to pass through the final shaft passages because of the presence of the disks, which enables improved sealing of those stages and thus improved performance of the pump, in particular by enabling reduction of the limit vacuum pressure.

The vacuum pump may further include one or more of the features that are described hereinafter, separately or in combination.

The thickness of the pumping profiles of the pair of rotors of the final stage is in particular less than the thickness of the pumping profiles of the pair of rotors of the preceding pumping stage.

For example, the thickness of the pumping profiles of the pair of rotors comprising a support disk is less than 6 mm. For each rotor comprising a support disk the greatest radial distance of the pumping profiles may be equal to or greater than the diameter of the support disk.

The pumping profiles may be of the lobe type (known as “Roots” profiles) or claw type (known as “Claw” profiles) or a combination of those types of embodiment depending on the stage. The vacuum pump is therefore a Roots pump for example. Each pumping profile can then for example include at least two lobes, such as between two and six lobes.

The vacuum pump may include more than two stages, that is to say at least three pumping stages mounted in series, for example three, four, five, six, seven, eight, nine or ten stages.

Each rotor of the pair of the penultimate pumping stage upstream of the final pumping stage may comprise a support disk made in one piece with the pumping profile, the compression chamber of the penultimate pumping stage featuring two cylindrical cavities complementary to the pair of rotors, the pumping profile of each rotor of the pair of the penultimate stage being configured to turn in a first part of the cylindrical cavity and the support disk being configured to turn in a second part of the cylindrical cavity, the first and second parts of the cylindrical cavities being axially inverted one relative to the other, the first parts overlapping transversely so that the pumping profiles of the pair are able to cooperate with one another to pump a gas. The support disks of the pairs of rotors of the final two pumping stages may be arranged in a quincunx along the two rotation axes. This arrangement enables further increase of the conductance at the level of the shaft passages of the final two pumping stages, thus very significantly improving the seal and therefore the pumping performance at limit vacuum. The stator may be formed by half-shells.

The rotors and the shaft carrying the rotors may be made in one piece.

Brief description of the figures

Other particular features and advantages of the invention will emerge clearly from the following description thereof given by way of nonlimiting indication and with reference to the appended drawings, in which:

[Fig. 1] figure 1 is a diagrammatic view of a vacuum pump in accordance with a first embodiment.

[Fig. 2] figure 2 is a diagrammatic partial view from above in section on a plane parallel to the rotation axes of the vacuum pump from figure 1. [Fig. 3] figure 3 is a diagrammatic perspective view of the rotary shafts and rotors of the vacuum pump from figure 1. [Fig. 4] figure 4 is a diagrammatic perspective view of one half-shell of the stator of the vacuum pump from figure 1.

[Fig. 5] figure 5 is a perspective view of the rotors of the pair of rotors of the final pumping stage of the vacuum pump from figure 1. [Fig. 6] figure 6 is a perspective view of a compression half-chamber of the final pumping stage of the half-shell from figure 4.

[Fig. 7] figure 7 is a view analogous to figure 2 for a vacuum pump in accordance with a second embodiment.

Detailed description Throughout what follows the horizontal plane is defined by the plane passing through the rotation axes of the shafts. The transverse direction is the direction in the horizontal plane that is perpendicular to the axial rotation direction.

A diagrammatic view of a vacuum pump 1 in accordance with the invention is represented in figure 1. The vacuum pump 1, preferably configured to discharge the gases at atmospheric pressure, includes a stator 2 delimiting at least two pumping stages 3a-3d mounted in series and two rotary shafts 5a, 5b. Each shaft 5a, 5b carries at least two rotors 71a, 71b, 72a, 72b, each rotor extending in one of the pumping stages 3a-3d (figures 1 and 2).

In the example illustrated in figure 1 the vacuum pump 1 includes four pumping stages 3a, 3b, 3c, 3d mounted in series between a suction inlet 4 and a discharge outlet 6 of the vacuum pump 1 and in which a gas to be pumped is able to circulate (the direction of circulation of the pumped gases is illustrated by the arrows in figure 1). Of course, without departing from the scope of the invention the vacuum pump 1 could include more stages or fewer stages that four, that is to say for example two, three, five, six, seven, eight, nine or ten stages.

As can be seen better in figures 2 to 4, each pumping stage 3a-3d is formed by a compression chamber 9, 90 delimited by the stator 2 and receiving a pair of rotors 71a, 71b, 72a, 72b. The compression chambers 9, 90 respectively feature two cylindrical cavities 10 complementary to the pair of rotors 71a, 71b, 72a, 72b. The two cylindrical cavities 10 of each compression chamber 9, 90 overlap in the transverse direction (that is to say in the direction perpendicular to the rotation axis).

In known manner, each compression chamber 9, 90 comprises a respective inlet and a respective outlet, the successive pumping stages 3a-3d being connected in series one after the other by respective channels 8 connecting the outlet of the preceding pumping stage to the inlet of the following stage (figure 4).

In a vacuum pump 1 of this kind with a plurality of stages 3a-3d the first pumping stage 3a, the inlet of which communicates with the suction inlet 4, is also known as the “suction stage”. The final pumping stage 3d, the outlet of which communicates with the discharge outlet 6, is also known as the “discharge stage”, and the required discharge pressure is generally higher than the ambient pressure in which the vacuum pump 1 is installed, typically an atmospheric pressure around 1 bar (10 ⁵ Pa). In operation, the rotors 71a, 71b, 72a, 72b turn in a synchronized manner in opposite directions in each stage to drive a gas to be pumped in a circulation direction extending from the suction inlet 4 to the discharge outlet 6. During the rotation the gas aspirated from the inlet of the pumping stage 3a-3d is trapped in the pumping volume generated by the rotors 71a, 71b, 72a, 72b and the stator 2 and is then driven by the rotors 71a, 71b, 72a, 72b to the next stage.

The rotors 71a, 71b, 72a, 72b feature conjugate pumping profiles 11a, 11b, for example of the lobe type (known as “Roots” profiles) or claw type (known as “Claw” profiles) or in accordance with a combination of these types of embodiment depending on the stage. The vacuum pump 1 is thus for example a lobed vacuum pump. Each pumping profile 11a, 11b can then for example include at least two lobes, such as between two and six lobes. In the example illustrated in figure 3 the pumping profiles include two lobes having a section of “8” or “bean” shape. The pumping profiles 11a, 11b are offset angularly by 90°, the rotors 71a, 72a at the top in figures 1 and 3 being represented “horizontal” while the rotors 71b, 72b at the bottom in these figures are represented “vertical” in the compression chambers 9, 90.

The shafts 5a, 5b carrying the rotors 71a, 71b, 72a, 72b are driven by a motor in a drive part 13 of the vacuum pump 1 about a respective rotation axis A and B (figure 1). The shafts 5a, 5b are supported by bearings that are usually lubricated by a lubricant contained in a casing of the vacuum pump 1 and are synchronized by means of a mechanical coupling device that is also lubricated including two toothed wheels meshing one in the other. Of course, other actuation modes may be envisaged for driving each shaft 5a, 5b. By way of nonlimiting example, a different motor could drive each shaft 5a, 5b.

The stator 2 is for example formed by half-shells 2a, 2b assembled at a surface passing through the rotation axes A and B as represented in figure 1 or assembled at edge surfaces in the axial direction.

Moreover, a sealing means (not represented) through which the shafts 5a, 5b are also able to turn can in the usual manner isolate the drive part 13 from the dry pumping part of the stator 2. Finally, a check valve (not represented) may be arranged in the discharge outlet 6 to prevent the pumped gases returning into the vacuum pump 1.

In known manner, the power P consumed by a vacuum pump 1 depends on the torque C exerted on the rotors 71a, 71b, 72a, 72b and on their rotation speed w. The torque is equal to the sum of the torques 6C exerted on the rotors 71a, 71b, 72a, 72b of each stage 3a-3d. Each torque is a function of the force and of the distance to the point at which the force is exerted, or to be more precise each torque is a function of the pressure difference Dr between the upstream side and the downstream side of the pumping stage 3a-3d, of the projected section of the rotors 71a, 71b, 72a, 72b and of the thickness e1, e2, e3 of the pumping stage 3a-3d. The pressure differences Dr of the final pumping stages being the highest, the final pumping stages, that is to say the stages closest to the discharge outlet 6, are those that require the most electrical power. The invention has in particular for object to provide a new architecture for a vacuum pump 1 with two shafts 5a, 5b enabling reduction of its electrical power by reducing the thickness e1 of at least the final pumping stage 3d.

To this end, at least each rotor 72a, 72b of the pair of rotors of the final pumping stage 3d upstream of the discharge outlet 6 comprises a support disk 12 made in one piece with the pumping profile 11a, 11b (figures 3 and 5). The support disk 12 is coaxial with the rotation axis A, B of the respective rotor 72a, 72b.

The pumping profile 11a, 11b of each rotor 72a, 72b of the pair of the final stage 3d is configured to turn in a first part 10a of the cylindrical cavity 10 of the compression chamber 90 (figures 4 and 6). The support disk 12 of each rotor 72a, 72b of the pair is configured to turn in a second part 10b of the cylindrical cavity 10. The first and second parts 10a, 10b of the cylindrical cavities 10 are axially inverted relative to one another and the first parts 10a overlap transversely so that the pumping profiles 11a, 11 b of the pair of rotors 72a, 72b are able to cooperate with one another to pump a gas.

With a (monobloc) pumping profile 11a, 11b made in one piece with a support disk 12 it is possible to produce much thinner pumping profiles 11a, 11 b, the support disks

12 providing the mechanical strength of the entire rotors 72a, 72b. The thicknesses of the pumping profiles 11a, 11b can then be reduced and these thin profiles 11a, 11b can be produced accurately, at least in the final pumping stage 3d, which makes it possible to be able to reduce the electrical power of the vacuum pump 1. Also, the cavities 10 of the compression chamber 90 of the final pumping stage 3d are enlarged because they receive the support disks 12. They are de facto easier to machine.

Another advantage is that this arrangement makes it possible to increase the conductances at the shaft passages of the final pumping stage 3d, which makes it possible to reduce gas leaks through the shaft passages, between the shafts 5a, 5b and the orifices in the transverse walls of the compression chambers 90. In fact, the support disks 12 of the rotors 72a, 72b force the leaking gases to circumvent the edges of the disks 12 rather than to flow along the shafts 5a, 5b. It is then more difficult for the gas to pass through the final shaft passages because of the presence of the disks 12, which makes it possible to improve the seal of those stages and thus to improve the performance of the pump 1, in particular by making it possible to reduce the limit vacuum pressure.

The thickness (axial dimension) e1 of the pumping profiles 11a, 11 b of the pair of rotors 72a, 72b of the final pumping stage 3d is advantageously less than the thickness e2 of the pumping profiles 11a, 11b of the pair of rotors 71a, 71b of the preceding pumping stage 3c. The thickness e1 of the pumping profiles 11a, 11 b of the pair of rotors 72a, 72b of the final pumping stage 3d is for example between 1.5 mm and 3 mm, such as 2 mm.

Moreover, all the thicknesses of the rotors 71a, 71b and of the pumping chambers 9 can decrease along the pumping stages, the pumping stage 3a on the suction inlet 4 side receiving the rotors 71a, 71b with the largest axial dimension. The thickness of the disk 12 is advantageously the smallest possible thickness so as not to increase excessively the overall size of the vacuum pump 1 whilst providing sufficient mechanical support for the pumping profile 11a, 11b. This thickness depends on the force, on the speed, and on the machineability of the material used. The thickness of the disk 12 is for example between 3 mm and 5 mm, such as 4 mm. The rotors 72a, 72b are for example made of cast iron. All the rotors 71a, 71b, 72a, 72b and the associated shaft 5a, 5b carrying the rotors 71a, 71b, 72a, 72b may be made in one piece. The vacuum pump 1 then includes two monobloc shaft-rotors. It can also be envisaged that the rotors 71a, 71b having no support disk 12 and the associated shaft 5a, 5b are made in one piece and that the rotors 72a, 72b having support disks 12 are mounted on a respective shaft 5a, 5b. In this latter case, two rotors 72a, 72b may be produced having virtually identical support disks 12, one being inverted relative to the other on its respective shaft 5a, 5b.

The greatest radial distance d of the pumping profile 11a, 11 b is for example equal to the diameter of the support disk 12 as illustrated in figure 5. The pumping profiles 11a, 11b are for example machined from the face of a cylinder of material with the diameter of the support disk 12. It may equally be envisaged that the greatest radial distance d of the pumping profiles 11a, 11b is greater than the diameter of the support disk 12. The pumping profile 11a, 11b is then no longer supported by the disk 12 at its edges. It is also possible for the greatest radial distance d of the pumping profiles 11a, 11 b to be less than the diameter of the support disks 12 but grooves must then be provided in the shafts 5a, 5b to allow the rotation of the support disks 12.

Figure 7 shows a second embodiment. ln this example, each rotor 72a, 72b of the pairs of the final two pumping stages 3c, 3d upstream of the discharge outlet 6, that is to say the final pumping stage 3d and the penultimate pumping stage 3c, comprise a support disk 12 made in one piece with the pumping profile 11a, 11b. The pumping profiles 11a, 11b of each rotor 72a, 72b of the pairs of the final two stages 3c, 3d are configured to turn in the first parts 10a of the cylindrical cavities 10 of the respective compression chambers 90. The support disks 12 are configured to turn in the second parts 10b of the cylindrical cavities 10, the first and second parts 10a, 10b of the cylindrical cavities 10 being axially inverted one relative to the other, the first parts 10a overlapping transversely so that the pumping profiles 11a, 11b of the pairs are able to cooperate with one another to pump a gas.

The thickness (axial dimension) e1 of the pumping profiles 11a, 11 b of the pair of rotors 72a, 72b of the final pumping stage 3d is advantageously less than the thickness e2 of the pumping profiles 11a, 11 b of the pair of rotors 72a, 72b of the preceding pumping stage 3c. The thickness e2 of the pumping profiles 11a, 11b of the pair of rotors 72a, 72b of the penultimate pumping stage 3c is advantageously less than the thickness e3 of the pumping profiles 11a, 11b of the pair of rotors 71a, 71b of the preceding pumping stage 3b. The thickness e2 of the pumping profiles 11a, 11b of the pair of rotors 72a, 72b of the penultimate pumping stage 3c is for example between 4 mm et 6 mm, such as 5 mm. The disks 12 of the pairs of rotors 72a, 72b of the final two pumping stages 3c, 3d are for example arranged in a quincunx along the two rotation axes A, B (figure 7). This arrangement enables further increase of the conductance at the level of the shaft passages in the final two pumping stages 3c, 3d, very significantly improving the seal and therefore the pumping performance at limit vacuum. The vacuum pump 1 may include more final pumping stages 72a, 72b configured in this way, that is to say more than two, the number of stages configured in this way being able to increase as the number of pumping stages of the vacuum pump 1 increases.