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Title:
COMBINATION GAS BEARING
Document Type and Number:
WIPO Patent Application WO/2015/032425
Kind Code:
A1
Abstract:
The present invention relates to a combination bearing (1) for a rotor in rotating machines, provided with a main bearing (6) and an auxiliary bearing (7), wherein the main bearing comprises a combined gas bearing and seal and wherein the auxiliary bearing is adapted to support the rotor during transient periods at start-up, shut-down or accidental operational loads, and wherein the main bearing and the auxiliary bearing are integrated within the same bearing housing (2).

Inventors:
BRENNE, Lars (Skarahødden 75, Sandnes, N-4236, NO)
BJØRGE, Tor (Nedre Stibakken 25 A, Hundhamaren, N-7564, NO)
MAZZALI, Carlo (Bernt Balchens vei 46, Fornebu, N-1364, NO)
UNDERBAKKE, Harald (Alveveien 18, Sandnes, N-4314, NO)
KIBSGAARD, Svend Tarald (Storgata 206, Etg. 1 - Leilighet H 01, Porsgrunn, N-3902, NO)
Application Number:
EP2013/068259
Publication Date:
March 12, 2015
Filing Date:
September 04, 2013
Export Citation:
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Assignee:
STATOIL PETROLEUM AS (Forusbeen 50, Stavanger, N-4035, NO)
International Classes:
F16C17/20; F16C32/04; F16C32/06; F16C33/10; F16C33/74; F16C39/02; F16C39/06
Foreign References:
US20110052375A12011-03-03
US4726694A1988-02-23
GB1302918A1973-01-10
US5017023A1991-05-21
US20120163742A12012-06-28
US5519274A1996-05-21
US3603654A1971-09-07
US4545586A1985-10-08
US4394091A1983-07-19
US5310265A1994-05-10
US20110052375A12011-03-03
US20120163742A12012-06-28
Attorney, Agent or Firm:
JACKSON, Robert (Dehns, St Bride's House10 Salisbury Square, London EC4Y 8JD, GB)
Download PDF:
Claims:
Claims

1. A combination bearing (1 ) for a rotor in rotating machines, provided with a main bearing (6) and an auxiliary bearing (7), wherein the main bearing comprises a combined gas bearing and seal, and wherein the auxiliary bearing (7) is adapted to support the rotor during transient periods at start-up or shut-down of the operation, and wherein the main bearing (6) and the auxiliary bearing (7) are integrated within the same bearing housing (2). 2. A combination bearing according to claim 1 or 2 wherein the bearing housing (2) is manufactured in one-piece.

3. A combination bearing according to any of the previous claims wherein the auxiliary bearing (2) is provided on a gas flow inlet side F of the main bearing (1 ).

4. A combination bearing according to any of the previous claims wherein the auxiliary bearing is provided on a gas flow outlet side of the main bearing (1 ).

5. A combination bearing according to any of the previous claims wherein the main bearing is provided with a first main bearing surface (9A) and a second main bearing surface (9B) and the auxiliary bearing (7) is arranged between the first main bearing surface and the second main bearing surface.

6. A combination bearing according to any of the previous claims wherein the main bearing surface (9,9A,9B) is provided with an uneven surface structure (8) such as a honeycomb structure or pattern of holes.

7. A combination bearing according to any of the previous claims wherein the combination bearing is provided with flow guiding means (13).

8. A combination bearing according to claim 7 wherein the flow guiding means (13) are provided on a rotor contact surface (10) of the auxiliary bearing (7) directed towards the rotor.

9. A combination bearing according to any of claims 7-8 wherein the flow guiding means comprises grooves (13).

10. A combination bearing according to any of claim 7-9 wherein the flow guiding means (13) are adapted such that the pressure drop is minimized over the auxiliary bearing (7) during operation of the main bearing (6). 1 1 . A combination bearing according to any of the previous claims wherein the main bearing clearance 5M is gradually increasing throughout the bore (4) in direction of higher pressure (P2).

12. A combination bearing according to any of the claims 1 -10 wherein the main bearing clearance 5M is gradually decreasing throughout the bore (4) in direction of higher pressure (P2).

13. A combination bearing according to any of the claims 1 -10 wherein the main bearing clearance 5M is constant throughout the bore (4).

14. A combination bearing according to any of the previous claims wherein the auxiliary bearing (7) comprises a ball bearing (12).

15. A combination bearing according to any of claims 1-13 wherein the auxiliary bearing (7) comprises a cylindrical bushing (14). 16. A combination bearing according to any of the previous claims wherein the auxiliary bearing (7) is provided with means for mist lubrication (1 5).

17. A combination bearing according to claim 17 wherein the cylindrical bushing (14) is provided with antifriction material.

18. A combination bearing according to claims 15-18 wherein the cylindrical bushing (14) is provided with additional back-up bearings comprising externally or internally arranged permanent magnets.

19. A combination bearing according to any of the previous claims wherein the auxiliary bearing (7) comprises a foil bearing (16) having at least one layer of foils arranged about the rotor (3).

20. A combination bearing according to claim 19 wherein the auxiliary (7) bearing comprises a foil bearing having several circumferential sectors (18) that are arranged to be variably positioned with respect to the rotor. 21 . A combination bearing according to any of the previous claims wherein the auxiliary bearing (7) comprises a MEG lubricated sleeve.

22. A combination bearing according to any of the previous claims wherein the auxiliary bearing (7) comprises a tilt pad bearing sleeve.

23. A combination bearing according to any of the previous claims wherein the auxiliary bearing (7) comprises a porous metal sleeve and that gas is injected through the porous metal sleeve to create a gas film to support for the rotor (3) when the main bearing (7) is not in operation.

24. A combination bearing according to any of the previous claims having additional back-up bearings comprising a passive permanent magnetic bearing which integrated in the combination bearing or arranged separately adjacent thereto. 25. A combination bearing according to any of the previous claims wherein the combination bearing (1 ) is a radial bearing formed as a cylindrical disc on the rotor which bears against an associated portion of the stator, whereby a gas film may be formed with stiffness and damping.

26. A combination bearing according to any of the previous claims wherein the combination bearing is an axial bearing formed as a cylindrical disc on the rotor which bears against an associated portion of the stator, whereby a gas film may be formed with stiffness and damping.

27. A combination bearing according to any of the previous claims wherein the bearing comprises at least two auxiliary bearings (7) arranged in connection with a main bearing (6) and said two auxiliary bearings are of a type suitable for withstanding contact with the rotor for a brief period during start-up, run-down or accidental operational loads.

28. A combination bearing according to any of the previous claims wherein the outside of the bearing housing 2 is stiff and provided with dampening means.

Description:
Combination bearing

Field of invention The present invention relates to the field of gas bearings. In particular the present invention relates to combined gas bearings and seal.

Background A combined gas bearing and seal can be efficiently used for supporting rotors both in radial and in thrust directions in rotating machines. The combined gas bearing and seal is configured to carry the load of a rotor during the normal operation of the rotor.

The documents US201 1 /0052375 and US2012/0163742 relate to combined bearing and seal solutions for both radial and thrust directions in rotating machines.

US201 1 /0052375 describes a combined gas bearing and seal having a sleeve seal with plain or textured static surface. The annular clearance has a geometric configuration that achieves sufficient stiffness and damping in the relevant frequency ranges provided that a pressure differential across the component is built up in order to generate static carrying capabilities. However during short periods of start-up or shut-down the differential pressure of the combined gas bearing and seal is not enough to guarantee a static carrying capacity of the rotor, and the combined gas bearing and seal is not designed to withstand severe wear or rub. Therefore, it is necessary to provide an auxiliary bearing as back-up bearing in addition to the combined gas bearing and seal.

To overcome this problem document US201 1 /0052375 proposes to mount permanent magnet bearing or arrange a hydrostatic lift generated by an accumulator adjacent the combined gas bearing and seal as a special start-up and run-down bearing. There is a need to further improve the combined gas bearing and seal and the back-up bearing to achieve a compact and reliable bearing which is configured to carry the load of a rotor both during dynamic operation and during short transient periods of start-up, shut-down or accidental operational loads. Summary

The main object of the present invention is therefore to provide an improved combination bearing for a rotor in rotating machines, which is configured to carry the load of the rotor both during dynamic operation and during the short transient periods of start-up, shutdown or accidental operational loads.

This object is achieved by the combination bearing defined in independent claim 1. Preferred embodiments of the invention will be understood from the dependent claims and the following description.

The combination bearing according to the present invention comprises a main bearing and an auxiliary bearing. The main bearing comprises a combined gas bearing and seal which operates to carry the load of a rotor during dynamic operation, and the auxiliary bearing operates as a back-up bearing carrying the load during the short transient periods of start-up, shut-down or accidental operational loads and when the main bearing is not in operation. The combination bearing according to the invention has a simple compact design which efficiently provides support of the rotor during periods when the main bearing is inactive. The compact bearing design allows the rotor to be made shorter and stiffer for enhanced rotor-dynamic performance, weight and cost reduction.

In particular the invention proposes that the main bearing and the auxiliary bearing are arranged in very close proximity, preferably the main bearing and the auxiliary bearing are integrated in the same bearing housing. Preferably the bearing housing is manufactured in one-piece, such that the main bearing and the auxiliary bearing are formed in same housing. This is advantageous in that the main bearing and the auxiliary bearing can be concentrically arranged and allows that the bearing clearance in the main bearing can be minimized which improves the performance of main bearing, the combined gas bearing and seal.

The geometry of a gas bearing and seal influences the static stiffness and the damping values of the main bearing. There are different types of gas bearings, depending on the geometry of the bearing clearance arranged between the stator and the rotor. According to the invention, the bearing clearance of the main bearing, the gas bearing and seal can be axially constant throughout the part of the bore where the main bearing is located or alternatively the bearing clearance may be axially convergent or divergent in direction of the gas flow. The magnitude of convergence or divergence is very small, the diameter of the bore varies from inlet to outlet approximately 0.5 % or less. The bearing clearance of the auxiliary bearing is constant. The main bearing surface has a minimum diameter which is always larger than the diameter of the rotor contact surface of the auxiliary bearing. Preferably the rotor and stator are coaxially or eccentrically arranged. The present invention also relates to a combination bearing having a main bearing formed as a gas bearing and seal, and an auxiliary bearing being suitable for both radial and thrust directions in rotating machines.

The present invention provides several alternative embodiments which achieves the described object. The alternative embodiments provide alternative combination bearings having different auxiliary bearing concepts that can be applied to both radial and axial gas bearings and seal. The auxiliary bearing is provided with a rotor contact surface which is adapted to be in contact with the rotor during transient periods of startup, shut-down or accidental operational loads. It is advantageous to provide the rotor contact surface with flow guiding means to ensure

Further advantages will be described in connection with different embodiments of the invention. Brief Description of Drawings

The present invention will now be discussed in more detail with the aid of preferred illustrative embodiments shown in the drawings, in which: Figure 1A shows a schematic sectional view of a basic structure of a combination bearing according to one embodiment of the present invention;

Figure 1 B shows a schematic sectional view of a basic structure of a combination bearing according to another embodiment of the present invention; Figure 1 C shows a schematic sectional view of a basic structure of a combination bearing according to another embodiment of the present invention; Figure 2 shows a schematic sectional view of a basic structure of a combination bearing according to another embodiment of the present invention,

Figure 3 shows a schematic sectional view of a basic structure of a combination bearing according to another embodiment of the present invention,

Figure 4 shows a schematic view of a combination bearing to another embodiment of the present invention wherein the auxiliary bearing is a ball bearing according,

Figure 5 shows a schematic view of a combination bearing according to another embodiment of the present invention wherein the auxiliary bearing is a ball bearing provided with flow guiding means,

Figure 6 shows a schematic view of a combination bearing according to another embodiment of the present invention wherein the auxiliary bearing is a cylindrical bushing provided with flow guiding means,

Figure 7 shows a schematic view of a combination bearing according to another embodiment of the present invention wherein the auxiliary bearing is a cylindrical bushing provided with flow guiding means,

Figure 8 shows a schematic view of a combination bearing according to another embodiment of the present invention wherein the main bearing is provided with a first and a second main bearing surface, Figure 9 shows a schematic view of a combination bearing according to another embodiment of the present invention wherein the main bearing is provided with a first and a second main bearing surface, Figure 10A-C shows a schematic cross sectional view of a combination bearing according to another embodiment of the present invention wherein the auxiliary bearing is a foil bearing.

Detailed description of the invention and embodiments thereof

With reference to the figures, the present invention shall be explained in more detail in connection with rotating machines, such as a compressors, pumps, turbines and expanders. Furthermore, it should be noted that the figures only show details which are important for the understanding of the invention. Moreover, same features are indicated by the same reference number throughout the specification. Furthermore it should be noted that the dimensions and proportions of illustrated embodiments and examples are intended for descriptive purpose only.

With reference to figure 1 A a schematic sectional view of a basic structure of one embodiment of the combination bearing 1 according to the invention is shown. The combination bearing 1 comprises a stator, an approximately cylindrical bearing housing 2, i.e., the stationary part surrounding a rotor 3. The bearing housing 2 is formed with a bore 4, whereby an annular clearance 5 is formed between the bearing housing 2 and the rotor 3. The bearing housing thus constitutes a "bearing point" for the rotor 3. The combination bearing according to the invention comprises a main bearing 6 and an auxiliary bearing 7. In this embodiment the bearing clearance 5 M of the main bearing and the bearing clearance 5 A of the auxiliary bearing 7 is constant throughout the bore 4.

The geometry of a gas bearing influences the static stiffness and the damping values. There are different types of gas bearings, depending on the geometry of the bearing clearance arranged between the stator and the rotor. According to the invention, the bearing clearance 5 M of the main bearing 6 (the combined gas bearing and seal) can be axially constant throughout the part of the bore 4 where the main bearing is located or alternatively the bearing clearance 5 M may be axially convergent or divergent in the direction of the gas flow throughout the part of the bore 4 where the main bearing is located. The magnitude of convergence or divergence is approximately 0.5 % or less, thus the bore diameter D of the bearing housing 2 in the region of the main bearing 6 varies typically less than 0.5% from the inlet to the outlet of the main bearing. The rotor and stator are preferably coaxially or eccentrically arranged. According to the invention, the bearing clearance 5 A of the auxiliary bearing 7 is constant throughout the part of the bore 4 where the auxiliary bearing is located, such that a rotor contact surface 10 of the auxiliary bearing 7 is parallel to the rotor axis 3. The rotor contact surface 10 has a diameter referred to as d in the figures. The main bearing 6 has a main bearing surface 9 having a diameter referred to as D in the figures. The minimum diameter D of the main bearing surface 9 is always larger than the diameter d of the rotor contact surface 10 of the auxiliary bearing 7.

The main bearing 6 comprises a combined gas bearing and seal. The main bearing 6 is provided with a seal sleeve 8 which forms a main bearing surface 9 in the bearing housing 2 surrounding the rotor 3. Preferably the sleeve seal 8 has an uneven surface structure such as a honeycomb structure or pattern of holes to achieve efficient sealing effect which can be seen in figure 4-9. With reference to figure 1 B a schematic sectional view of a basic structure of another embodiment of the combination bearing 1 according to the invention is shown, wherein the bearing clearance 5 M has a gradually increasing diameter D in the direction of higher pressure (P2). The auxiliary bearing 7 is located on the inlet side, the high pressure side (P2), of the bearing housing 2. The minimum diameter D of the main bearing surface 9 is always larger than the diameter d of the rotor contact surface 10 of the auxiliary bearing 7.

The pressure difference is used, i.e., the pressure drop across the clearance, to obtain the function as a combined gas bearing and seal. The stiffness is provided by allowing the annular clearance to converge towards the lower pressure, such that the inlet clearance is greater than the outlet clearance. Positive direct stiffness is thus obtained in the combined gas bearing and seal. Positive direct damping of gas rotation in the annular clearance may be provided by means of the characteristics of the main bearing surface 9 facing the rotor.

Fig. 1 C shows a schematic sectional view of a basic structure of another embodiment of the combination bearing 1 according to the invention wherein the the bearing clearance 5 M has a gradually decreasing diameter D in the direction of higher pressure (P2).

It should be noted that the convergence and divergence of the main bearing surface 9 showed in Fig. 1 B and Fig.l C is very exaggerated for the purpose of illustration. In reality the magnitude of convergence or divergence is very small in relation to the diameter D of the main bearing surface 9. Furthermore, it should be noted that the figures of the application are schematic illustrations and some of the illustrated embodiments indicate that the main bearing clearance 5 M is constant. However all embodiments of the combination bearing according to the present invention may be provided with a convergent, divergent or constant bearing clearance 5 M of the main bearing as previously described.

Figure 2 shows an alternative embodiment wherein the main bearing 6 is provided with a first main bearing surface 9A and a second main bearing surface 9B.The first and second main bearing surfaces 9A,9B are separated in the axial direction and arranged apart from each other. This allows for the auxiliary bearing 7 to be positioned, arranged, between the first and the second main bearing surface 9A and 9B.

The auxiliary bearing 7 is adapted to support the rotor 3 during transient periods at start-up or shut-down of the operation and when the main bearing 6 is inactive and not in operation. During machine shut down the main bearing 6, the combined gas bearing and seal, should be able to support and sustain the rotor weight for a short time before the unit speed reduction will cause a pressure decrease on the system that will consequently reduce the gas bearing load carrying capabilities. When the load carrying capability of the main bearing 6, the combined gas bearing and seal, is lower than a minimum of load carrying capability necessary for lifting the rotor, this gives way to the landing of the rotor on an auxiliary bearing 7. Once the main bearing 6 is not able to support the rotor load the rotor lands on the rotor contact surface 10 provided on the auxiliary bearing 7.

The auxiliary bearing 7 is only in touch with the rotor 3 when the main bearing 6 is not in operation or during accidental loads that might exceed the main bearing load carrying capacity. During start-up the main bearing 6 has a lifting effect on the rotor 3 such that it is lifted from the supporting contact surface 10 of the auxiliary bearing 7.

The main bearing 6 and the auxiliary bearing 7 are integrated within the same bearing housing 2. The combination bearing 1 is therefore compact, which is advantageous since the space available within rotating machines is very limited. Furthermore by integrating the main bearing 6 and the auxiliary bearing 7 in the same bearing housing 2 the main bearing 6 and the auxiliary bearing 7 can be concentrically arranged with good accuracy. Preferably the bearing housing 2 is manufactured from one-piece together with main bearing 6 and auxiliary bearing 7. This has the effect that the bearing clearance 5 M can be minimized which improves the performance of the main bearing combined gas bearing and seal.

The outside of the stator and the bearing housing 2 is stiff and provided with dampening means such as O-rings, bump foils or squeeze film dampers to provide additional dampening during both normal operation and lifting/landing of the rotor on the auxiliary bearing/s. The dampening device, if employed, could also allow some angular movement in order to take advantage of the restoring moment inherent in a convergent seal/bearing. This could help reduce angular alignment requirements.

The auxiliary bearing 7 may be arranged on the gas flow inlet side of the combination bearing in the flow direction F, as shown in figure 1A-1 C. Alternatively the auxiliary bearing 7 may be arranged on the gas flow outlet side of the combination bearing 1 as shown in figure 3. As previously described, the auxiliary bearing 7 may alternatively be arranged between a first main bearing surface 9A and a second main bearing surface 9B in the axial direction depending on the type of auxiliary bearing 7, as shown in figure 2.

According to the invention the main bearing 6 can be combined with different types of alternative auxiliary bearings 7, thereby achieving the desired effect of supporting the rotor during transient periods when the bearing is not active. Several embodiments will be described in detail below.

With reference to figure 4, one embodiment of the combination bearing 1 according to the invention comprises a main bearing 6 which is a combined gas bearing and seal and an auxiliary bearing 7 comprising a ball bearing 1 1 as back up bearing integrated in the same bearing housing 2. The ball bearing 1 1 is preferably arranged on the flow inlet side, the high pressure side (P2) of the bearing housing 2. The ball bearing 1 1 is provided with an inner ring 12. The inner ring 12 of the ball bearing 1 1 comprises the rotor contact surface 10 which supports a rotor 3 (not shown) during the transient periods. Diameter d of the inner ring 12 is adapted to be less than the diameter D of the main bearing surface 9, where D corresponds to a minimum diameter of the main bearing surface 9. The ball bearing 1 1 may influence the gas flow, it is therefore necessary to minimize the pressure drop Δρ due to the auxiliary bearing 7. The contact surface 10 of the inner ring directed towards the rotor may therefore optionally be provided with flow guiding means, such as relief grooves. This is showed in figure 5 which shows another embodiment of the combination bearing 1 according to the invention . In this embodiment the auxiliary bearing comprises a ball bearing 1 1 having gas flow guiding means 13 arranged on the inner ring 12 directed towards the rotor. The gas flow guiding means 13 are provided as a pattern of grooves 13 separated by ribs 10 as shown in figure 5. The ribs 10 operate as rotor contact surface during transient periods of start-up and shut down of the rotor machine.

The depth of the grooves 13 is adapted such that the gas flows easily passed the grooves during operation of the main bearing. Typically the diameter d g across the bore between two oppositely arranged grooves is equal to or larger than the minimum diameter D of the adjacent main bearing surface. The diameter d across the bore between two oppositely arranged ribs 10 which operate as rotor contact surface is always smaller than the minimum diameter D of the adjacent main bearing surface to ensure that the rotor is carried by the auxiliary bearing during transient operations.

The depth and width of the grooves should be adapted such that the pressure drop across the ball bearing inner ring 12 is minimized while maintaining, guaranteeing, sufficient contact between the contact surface 10 of the inner ring 12 and the rotor during the transient periods.

Preferably the auxiliary bearing 7 with flow guiding means 13 is arranged upstream of the main bearing 6 with respect to the incoming flow F. The grooves may be adapted to direct the gas flow F to move in a preferred direction. Preferably the grooves 13 are arranged such that the circumferential component of the incoming gas flow F is optimized at the flow inlet side of the main bearing surface 9. This is advantageous in that the grooves 13 create a pre-swirl in the gas flow which is beneficial to the working principle of the main gas bearing 6 in that the dampening characteristics can be tuned. The grooves 13 are preferably arranged such that the gas flow is rotated in the opposite direction to the rotation of the rotor. Once the main bearing 6 is not able to support the rotor load, the rotor lands on the rotor contact surface 10 provided on the inner ring 12. During normal operation the ball bearing 1 1 is not free to rotate. However shortly after the load carrying capability of the main bearing 6 begins to decrease, the ball bearing 1 1 may already start spinning in the direction of the rotor due to the drag of the grooves 13. The speed differential between the rotor 3 and the inner ring 12 at landing is thereby reduced, with beneficial consequences on the heating and wear reduction.

I n another embodiment of the combination bearing 1 according to the invention the auxiliary bearing comprises a ball bearing 1 1 being arranged on the inlet side , the high pressure side, of the combination bearing and is further provided with an annular brush seal (not shown in figure) between the side surface of the inner ring 12 of the ball bearing and the side surface of the main bearing 6 to avoid that gas flows through the balls of the bearing and by-passing the flow guiding means 13 and the pre-swirl resulting from flowing via the flow guiding means arranged on the inner ring 12.

In another embodiment of the combination bearing 1 according to the invention the auxiliary bearing comprises a ball bearing 1 1 having mist lubrication (not shown in figures). The ball bearing 1 1 is provided with mist lubrication arranged within the bearing housing 2 to ensure slight lubrication of the rolling elements by using process gas locally cooled below the dew point.

To prevent rotation of the ball bearing inner ring 12 during normal operation once the rotor is no more in contact with the rotor contact surface 10, preloads of the balls 1 1 with respect to the running races can be envisioned (not shown in figure). As alternative, a permanent magnet can be embodied into the inner ring 12 so that an electromagnet installed on the stator can prevent the inner ring 12 to rotate during normal operation and release it before imminent shut down. With such latter device the preload on the balls 1 1 can be minimized with the benefits of lowering heat generation and wear. In another embodiment of the combination bearing 1 according to the invention the auxiliary bearing comprises a cylindrical bushing. The cylindrical bushing preferably comprises an antifriction material (i.e. peek) arranged within the bearing housing of the combination bearing . This is advantageous in that excessive rub from several start-stop events can be avoided. Preferably the cylindrical bushing is arranged on the inlet side of the bearing housing.

With reference to figure 6 a combination bearing 1 having a cylindrical bushing 14 is shown which is further provided with flow guiding means 13 , as previously described, on the inner diameter d of the contact surface 10 of the cylindrical bushing 14 in order to optimize the tangential component of the incoming flow.

Alternatively the main bearing 6 is provided with a first main bearing surface 9A and a second main bearing surface 9B and the cylindrical bushing 14 is provided between the first main bearing surface and the second main bearing surface in the axial direction, as shown in figure 8 and 9. In figures 8 and 9 it is further shown that the grooves 13 are flush with the main bearing surface 9,9A and 9B. Preferably the cylindrical bushing having flow guiding means is provided upstream of the main bearing 6, as shown in figure 6 and 7. The cylindrical bushing having flow guiding means may optionally be arranged between the first main bearing surface 9A and the second main bearing surface 9B in order to provide a tangential component to the gas flow as previously described. I n another embodiment of the invention the cylindrical bushing can be located downstream the main bearing 6 to prioritize short bearing width span for rotor dynamic purposes (not shown in figures).

It is advantageous to provide a cylindrical bushing 14 with flow guiding means 13 as auxiliary bearing since it is possible to produce the main bearing 6 and the bushing 14 in one piece, guaranteeing a very good level of concentricity between the two bearing, further allowing for better dynamic performances of the main bearing. The minimum diameter D of the main bearing 6 is always larger than the diameter d of the auxiliary bearing contact surface 10. Typically the diameter across the bore between two oppositely arranged grooves d g is equal to or larger than diameter D of the adjacent main bearing surface.

The cylindrical bushing 14 may optionally also be provided with means for mist lubrication 15 as shown in figures 7-9 to inject cooling fluid or lubrication to mitigate excessive heating and avoid seizure during machine operation. The fluid (i.e. MEG) can be envisioned to be injected through several injection openings15 in the contact surface 10 that will get in contact with the rotor. The rotor contact surface 10 may be further adapted to improve the spreading of the lubrication.

Another embodiment the combination bearing 1 provided with an auxiliary bearing comprising a cylindrical bushing is further provided with externally or internally arranged permanent magnet as additional back-up bearings (not shown in figures). This is advantageous in that the load carrying capability of the combination bearing and the additional back-up bearing reduces the specific pressure on the bushing.

With reference to figure 10A-10C a cross section of another embodiment of the combination bearing 1 according to the invention wherein the auxiliary bearing 7 comprises a foil bearing 16 provided with at least one layer of foils arranged in the bearing housing 2.

The layer of foils is arranged about the rotor 3 and is provided with contact surfaces 17 which support the rotor 3 when the main bearing 6 (not shown in figure), the gas bearing and seal is not active. The foil bearing 16 may be located anywhere within the bearing housing, however it is preferred to arrange the foil bearing 16 on the outlet side of the bearing housing 2 in the direction of the gas flow F to avoid pressure build up over the foil bearing 16 and to have easy access to the foils for maintenance.

In figure 10A-C it is shown that the foil bearing 16 is provided by arranging several circumferential sectors 18 that can vary their position with respect to the rotor 3 such that the circumferential sectors 18 can be separated from the rotor once a minimum required differential pressure is built up in the machine. This can be achieved by using a differential pressure driven system or alternatively an electromagnetic 19 (Fig. 10A,B) or hydraulic system 20(Fig. 10C) to automate this movement of the circumferential sectors 18 away from the rotor surface, thereby allowing the main bearing to fully support rotor.

One advantage of using a foil bearing 16 as auxiliary bearing 7 is that when the foils are inactive and the main bearing 6, the gas bearing and seal 6, is in operation, the foils 16 are removed from the annular clearance 5 A surrounding the rotor 3. The annular clearance 5 A is therefore larger in this case than when using other types of auxiliary bearings. This has the effect that pressure build up over the foils 16 is avoided. Other types of foil bearings of traditional type can also be envisioned as auxiliary bearing.

Another embodiment of the combination bearing 1 according to the invention is provided with an auxiliary bearing which comprises segments or a tilt pad bearing sleeve provided in the bearing housing . Preferably the combination bearing is provided with a first main bearing surface and a second main bearing surface as previously described, and the segments or the tilt pad bearing sleeve are/is provided between the first main bearing surface and the second main bearing surface ( not shown in figures). I n another embodiment of the combination bearing according to the invention the auxiliary bearing comprises a porous metal sleeve arranged within the bearing housing (not shown in the figures)The porous metal sleeve may be provided at the inlet side or the outlet side of the main bearing. Optionally the porous metal sleeve is provided between a first main bearing surface and a second main bearing surface as previously described. Preferably a gas film is created by injecting gas through the porous metal sleeve, such that the gas film supports the rotor 3 when the main bearing, the gas bearing and seal is inactive and not in operation . In addition, also the main bearing 6 can be manufactured of a porous material for a further integrated design. Another embodiment of the combination bearing 1 according to the invention is provided with an auxiliary bearing which comprises a MEG lubricated sleeve (not shown in figures). MEG is only injected as lubricant during start-up or shut down events when the main bearing, the gas bearing and seal is inactive. The auxiliary bearing is provided with openings 15 to supply the lubrication to the rotor contact surface 10, which can be seen in the figures.

Another embodiment of the combination bearing according to the invention the combination bearing comprises at least two auxiliary bearings arranged in connection with main bearing, and said two auxiliary bearings are of a type suitable for withstanding contact for a brief period during start-up or run-down as the previously described auxiliary bearings.

The different types of auxiliary bearings according to the different embodiments described above may be combined or coupled with other back-up solutions such as external feed accumulator or permanent magnetic bearing.

In one embodiment of the combination bearing according to the invention the combination bearing is provided as a radial bearing formed as a cylindrical disc on the rotor which bears against an associated portion of the stator, whereby a gas film may be formed with stiffness and damping.

In another embodiment of the combination bearing according to the invention the combination bearing is provided as an axial bearing formed as a cylindrical disc on the rotor which bears against an associated portion of the stator, whereby a gas film may be formed with stiffness and damping according to the same principle as in a radial bearing having desired dynamic stiffness and damping (not shown in figures).

The above-mentioned embodiments, variants and examples of the present invention may be freely combined within the limits of the following claims without departing from the invention.