The present invention concerns a device for absorbing an axial force exerted on a machine shaft supported on radial bearings, such as the shaft of a compressor, a motor, a turbine or the like.

In particular, the invention concerns a device for absorbing an axial force exerted on a machine shaft .

While such a machine is operational, varying axial and radial forces are exerted on the shaft of the machine, which makes it necessary to provide the machine with bearings which can absorb said forces .

Traditionally, the shaft is supported on radial bearings by means of two radial bearings provided at a certain distance from one another to ensure a good stability for the shaft and any rotor possibly provided upon the latter.

The shaft is axially loaded with a force whose magnitude varies in time and which may also change direction.

Conventionally, two bearings are applied, whereby one bearing absorbs the axial force in one direction, whereas the other bearing absorbs the axial force in the opposite direction. This means that at anytime only one of both bearings is loaded, while the other one remains unloaded.

Replacement sheet (rule 26) The machine shaft is supported on axial bearings in the known manner, whereby the shaft is provided with one or several elements which are necessary for the good functioning of the axial bearing. Traditionally, the aforesaid elements have the shape of an axial thrust disc or shoulder, whereby these elements have been provided or are placed on the side wall of the shaft, and whereby these elements function as an intrinsic part of the axial bearing.

As the machine's design provides larger loads, also larger bearings will be required, which implies that the shaft in such heavier loaded machines should be made sufficiently long to offer space to such larger bearings. Indeed, the shaft must be sufficiently long to provide the necessary space and room for an axial thrust disc, shoulder or the like on the one hand, and for the fixed part of the axial bearing itself on the other hand.

What precedes shows that the required length for the shaft increases as the axial bearing is made heavier.

Higher rotational speeds of the machine's shaft result in the use of active magnetic bearings and/or air bearings being preferred or, in many cases, even necessary.

A large speed of rotation of the shaft causes a large centrifugal force on the shaft and the radial bearings in case ^" of the slightest imbalance, making it necessary to balance the shaft before the machine is put into use. Even when the shaft is balanced, some imbalance still remains to a certain extent, as a result of which the shaft is excited by the centrifugal force while rotating.

As soon as the shaft reaches its critical rotational speed, the centrifugal force will excite the shaft with a frequency which, as is known, coincides with the first characteristic frequency of the shaft, resulting in resonance of the shaft.

When the shaft comes into resonance, the deviation and the deformation of the shaft is only limited by the damping in the system. However, magnetic bearings and air bearings produce very little damping, which would result in irreparable damage to the machine if no additional measures were taken in case of resonance.

From what precedes it is clear that the maximal rotational speed of the machine is restricted by the critical rotational speed of the shaft. The first characteristic frequency of this shaft concerns a bending frequency, whereby the length of the shaft is the most decisive factor. The first characteristic frequency of the shaft strongly decreases as the length of the shaft increases.

Heavy loaded machines which are conventionally supported on axial bearings in the above-described manner have the major disadvantage that the first characteristic frequency of the shaft of such a machine, and consequently the critical rotational speed of the shaft, tend to be relatively low, as a result of which the useful rotational speed of the machine is restricted. If the machine is a compressor, this will result in an output of the compressor concerned having a low energy density.

From German patent DE 10 2006 049 516, the use of a static gas bearing on a free shaft end at the back of an impeller is known.

The gas bearing is controlled by the measurement of force in a hydrodynamic tilting pad bearing, such that the forces in the hydrodynamic bearing are restricted by compensation in the gas bearing.

The control hereby provides in a closed loop system with feedback in order to reduce the force to zero. As far as control engineering is concerned, additional measures are thereby necessary, such as a low-pass filter, in order to guarantee a stable working under all circumstances.

Another disadvantage is that the gas bearing must be sealed. Given the high rotational speeds and large diameter, there is a large sliding speed in said sealing, as a result of which the sealing is subject to wear and leaks may prematurely occur.

The present invention aims to remedy one or several of the above-mentioned and/or other disadvantages by providing a device for absorbing an axial force exerted on a machine shaft, which shaft is provided with one or several rotors, and whereby this device comprises at least two axial bearings to support the aforesaid shaft, namely a first bearing and a second bearing, whereby the aforesaid first bearing is a primary bearing of the active magnet type which absorbs the major part of the aforesaid axial force and which is designed such that, for the mounting thereof, no mounting space on the shaft's length needs to be provided, whereby the aforesaid first bearing is controlled by means of a control unit and whereby the control unit is at least connected to a pressure sensor and comprises an algorithm to control the aforesaid first bearing on the basis of a measuring signal from the aforesaid pressure sensor.

This means that the aforesaid first bearing is preferably not controlled on the basis of an axial position measurement of the machine's shaft.

An advantage hereby is that it is easier to control the first bearing than to control a conventional axial active bearing.

In other words, according to the invention, the shaft must not be extended to place the bearing, or to rephrase it, the first bearing is designed such that it has no influence whatsoever on the shaft length that is required for the working of the machine.

The aforesaid first bearing hereby works in one direction and absorbs more than 75% of the total axial force in that particular direction, preferably at least 85% of the total axial force, and even more preferably at least 95% of the total axial force.

With a device according to the invention, the shaft can be made shorter than the shaft of an identical machine, supported in the conventional manner on axial bearings.

Indeed, according to the invention, the most heavily loaded and consequently the most sizeable bearing is provided in the device in such a manner that this bearing has no influence whatsoever on the length of the shaft that is required for the working of the machine, as a result of which the length of the shaft can be restricted to an absolute minimum. In other words, the aforesaid first bearing will not occupy any axial mounting space on the shaft while the device according to the invention is in use.

The reduced length of the shaft in the case of the invention offers several advantages. The first characteristic frequency and the critical rotational speed of said shaft increase, as a result of which the maximally permitted rotational speed of the machine will be higher than in the case of a machine with conventional bearings.

Another advantage is that the shaft has less mass, which allows savings on material and transport costs.

According to a preferred embodiment, the first bearing acts directly on a far end of the shaft or on a rotor which has been provided on the shaft or which is a part of it. What precedes implies that, for the good functioning of the first bearing, the shaft or rotor must not be provided with additional elements that are traditionally used, such as a thrust disc, shoulder or the like.

This offers the advantage that the shaft or rotor does not need any final processing to create a thrust disc, shoulder or the like in the shaft or rotor or to provide the necessary means for fixing the aforesaid elements to the shaft or rotor. As a result, the machine is less expensive to produce.

In a practical embodiment, the pressure sensor may be provided nearby the impeller, and the magnetic bearing is controlled by the measuring signal from the pressure sensor.

By replacing the gas bearing in the known devices by a magnetic bearing, a sealing to seal off the bearing is no longer necessary.

Another advantage is that no consumption of compressed air occurs .

Yet another advantage is that a pressure measurement allows to control the magnetic bearing in a simple, yet efficient manner. Indeed, the measured pressure is a major factor for the forces that are being absorbed by the magnetic bearing. The aforesaid control on the basis of pressure and the use of the magnetic bearing results in a direct compensation of the axial gas pressure forces, which leads to a stable system.

According to a preferred embodiment, the second axial bearing is also of the active magnetic type, whereby the first bearing is controlled as a function of a machine variable, for example, in the case of a compressor, the outlet pressure. The aforesaid machine variable that is used to this end is preferably a variable that varies little in time.

Another advantage is that a control on the basis of a simple open loop control system with a gain factor may be provided.

An advantage related thereto is that the primary bearing will not introduce any unstable conditions.

The present invention also concerns a centrifugal compressor provided with a shaft with one or several rotors and with a device for absorbing an axial force that is exerted on the aforesaid shaft, which device comprises at least two axial bearings supporting the aforesaid shaft, a first bearing and a second bearing respectively, whereby the aforesaid first bearing is a primary bearing of the active magnetic type which absorbs the major part of the aforesaid axial force and which is made such that it does not affect the required length of the shaft, or, in other words, no mounting space must be provided on the shaft length for this first bearing, whereby the aforesaid first bearing is controlled by means of a control unit and whereby the control unit is at least connected to a pressure sensor and comprises an algorithm to control the aforesaid first bearing on the basis of a measuring signal from the aforesaid pressure sensor.

The advantages related to a centrifugal compressor according to the invention are identical to the above- discussed advantages offered by a device according to the invention.

In order to better explain the characteristics of the invention, the following preferred embodiments of a centrifugal compressor, provided with a device according to the invention, are described by way of example only without being limitative in any way, with reference to the accompanying drawings, in which:

figure 1 schematically shows a section of a centrifugal compressor provided with a conventional device for absorbing an axial force; figure 2 shows a variant of figure 1; figure 3 schematically shows a section as that in figure 1, but for a centrifugal compressor provided with a device according to the invention; figure 4 shows a variant of figure 3.

The figures represent the shaft 1 of a machine which is in this case made as a centrifugal compressor, whereby said shaft 1 is provided with a rotor 2 in the shape of an impeller on one far end. The shaft 1 and rotor 2 are mounted in a rotating manner in a housing 3 of the compressor.

The shaft 1 is provided with two radial bearings 4. The shaft 1 is driven by an electromotor, as represented in figure 1, whereby said electromotor is made as a high-speed permanent-magnet motor in the present example.

The shaft 1 in figure 1 is radially supported by two radial bearings 4, whereas the axial forces, introduced during working of the machine, are absorbed by two axial bearings β. The axial as well as the radial bearings 6, 4 respectively, are in this case active magnetic bearings.

The aforesaid axial bearings 6 act on a shoulder 7 of the shaft 1, namely on the radially directed side flanges of a shaft part having a larger diameter than the rest of the shaft 1.

It is clear that for the axial bearings 6, a certain shaft length should be provided, in other words that mounting space should be provided on the perimeter of the shaft so as to make room for said axial bearings 6, which is disadvantageous, as explained in the introduction, to the maximally permitted rotational speed with respect to the risk of resonance phenomena occurring.

In the case of figure 2, the two axial bearings 6 are represented in a configuration whereby these bearings 6 act on an axial thrust disc 8 which is fixed to the shaft 1 or which is part of it, and whereby these bearings 6 also make it necessary for the shaft 1 to have a certain length.

Figure 3, however, shows a device according to the invention, whereby two bearings are applied, namely a first bearing 9 which serves as a primary axial bearing and which is provided on a free end of the shaft 1, and a second bearing 10 in the shape of a secondary axial bearing which is in this case made as a double-acting bearing extending on either side of an axial thrust disc 8 on the shaft 1.

The bearings 9 and 10 which are represented in the figures are in this case of the active magnet type. According to the invention, it is not strictly necessary that the second bearing is of the active magnet type.

The working of a device according to the invention is simple and as follows.

The shaft 1 is driven by the motor 5, whereby the rotor 2 at the far end of the shaft 1 rotates in the housing 3. As a result, the sucked-in air is compressed in the known manner, after which the compressed air is discharged to a downstream application.

Given the high rotational speeds of the shaft 1, bearings of the active magnetic type are preferably used. The working of an active magnetic bearing is known, and it uses amongst others an electromagnet, amplifiers and sensors to support the shaft 1 in a contactless way. While the machine is working normally, the shaft/rotor system 1, 2 is loaded with an axial force in the direction of arrow A. This axial force mainly consists of a nominal, almost static component, caused by the pressure distribution on the rotor 2.

The first axial bearing 9 is dimensioned such that it can absorb this nominal, almost static axial force. Consequently, this first bearing 9 must be made relatively large and heavy.

Traditionally, as represented in figures 1 and 2, the axial bearings 6 act on an axial thrust disc 8, on a shoulder 7 or on other elements which are provided on the shaft 1. It is clear that in such cases, the shaft 1 must be made sufficiently long so as to provide sufficient room for the axial bearings 6.

According to the invention, the first, primary bearing 9 is made such that it does not affect the required length of the shaft 1, as is illustrated for example by figure 3 in which the first bearing 9 acts directly on the free end of the shaft 1.

In this way, the first bearing 9 can be placed in the extension of the shaft 1, at its free end, such that no mounting space must be provided on the shaft length for said bearing 9, and consequently the shaft 1 must not be extended so as to position the bearing 9. In other words, the placement of the bearing 9 according to the invention has no effect whatsoever on the shaft length that is required for the operation of the machine.

While the machine is operational, small force fluctuations also arise which are axially directed according to arrow A or arrow B. These small force components are absorbed by the second, secondary axial bearing 10.

The second bearing 10 is realised in the known manner, whereby this bearing 10 uses at least one shoulder 7, axial thrust disc 8 or the like, provided on the shaft 1, whereby the aforesaid shoulder, thrust disc or the like is an intrinsic part of said second bearing 10.

In this case, the second bearing 10 consists of two active magnetic bearings which each act on an axial thrust disc 8 provided along the perimeter of the shaft 1. In the given example, use is made of a single thrust disc 8 on which the two active magnetic bearings of the second bearing 10 act on either side.

As the second bearing 10 is minimally loaded, the size of this bearing can be restricted, as a result of which the effect on the length of the shaft 1 is restricted.

Consequently, with a device according to the invention, the length of the shaft 1 can be restricted compared to the embodiment described in figures 1 and 2. As a result, the critical rotational speed of the shaft 1 increases, which makes it possible to make the shaft 1 rotate at higher rotational speeds with the same impeller 2 design and with the same materials being applied, and consequently to deliver higher pressures and more power.

Figure 4 shows a variant according to the invention whereby the first bearing 9 acts directly on the back side of the impeller 2. The back side of the impeller 2 is the flat side of the rotor, which is the side without any blades. The second bearing 10 in this case consists of two axial bearings which are each positioned at a shoulder 7 provided on the shaft 1.

Given the restricted load of the second bearing 10, this second bearing 10 can be made small, thus limiting the impact on the length of the shaft 1.

According to the invention, the first bearing 9 is controlled as a function of a machine variable whereby use is made of a control unit 11 comprising an algorithm to control the bearing 9 on the basis of a measuring signal from a pressure sensor 12. Preferably, the aforesaid pressure sensor 13 measures a variable that varies little in time. An example of a relevant variable which varies little in time that can be used to this end is the prevailing pressure in the compressor, in particular the outlet pressure of the compressor. Indeed, this pressure forms the major contribution to the total axial force. In the given example, the aforesaid pressure sensor 12 is provided to that end on the flat side of the impeller, which is free of blades. The second bearing 10 is preferably controlled by a control unit 13 which is connected to a position sensor 14 to determine the axial position of the shaft 1.

Although active magnet bearings are used in the preceding embodiments, also an air bearing can be used for the second bearing.

Other embodiments than those discussed above with reference to figures 3 and 4 are possible as well, while still remaining within the scope of the invention. Thus, for example, the first bearing 9 may act on the back side of the impeller 2 while the second bearing 10 makes use of an axial thrust disc 8 provided along the perimeter of a shaft 1.

Another possible embodiment within the scope of the invention makes use of a first bearing 9 which acts on a shaft end of the shaft 1 with a second bearing 10 which acts via two shoulders 7 on the shaft 1.

In the above-described embodiments, the device according to the invention is always applied in a centrifugal compressor, in particular in a turbo compressor, but a device according to the invention can also be applied in other machines which are provided with a shaft which is subject to axial forces, such as for example a turbine, a motor or the like.

The present invention is by no means restricted to the embodiments described by way of example and represented in the accompanying drawings; on the contrary, a device according to the invention for absorbing an axial force exerted on a machine shaft and a centrifugal compressor provided with such a device can be made in all sorts of shapes and dimensions while still remaining within the scope of the invention.