Turbomachine with a shaft connected to an impeller by means of an axially sandwiched friction link - Patents.com

Abstract

A turbomachine comprising a shaft (10) having a first end and an impeller (20) disposed on and coupled to the first end of the shaft, the turbomachine comprising a friction ring (30) clamped between an axial surface (13) of the first end of the shaft (10) and an opposing axial surface (23) of the impeller (20). Preferably, the friction ring (30) comprises a surface having hard particles, for example of diamond or ceramic, to increase friction between and couple the clamped parts.

Description

[Technical field]

[0001] The present invention relates to turbomachines. [Background technology]

[0002] A turbomachine is a machine in which energy is exchanged between a fluid flow and one or more impellers disposed and coupled to a shaft, the shaft and impellers being arranged to rotate together about an axis of rotation.

[0003] Turbomachines often only have one impeller, which is placed in an overhanging manner and connected to the axial plane (end face) of the machine shaft by a bolted connection. This connection presents challenges, for example the need to transmit large torques with limited joints. Furthermore, the overhanging mounting, with bearings only on one axial side of the impeller, combined with the very high machine speeds, which can reach tens of thousands of revolutions per minute, places very high demands on the alignment of the impeller and the shaft in order to minimize unbalance of the rotating masses.

[0004] Particularly during start-up of a turbomachine, the opposing end faces of the impeller tend to move relative to the end faces of the shaft, which can increase the imbalance of the rotating masses and cause unacceptable levels of vibration. This relative displacement can include radial relative displacement of the opposing end faces, tilting of the opposing end faces, torsional displacement of the opposing end faces, or a combination of two or more of these forms of displacement. This tendency for relative displacement increases when the impeller becomes heavier relative to the shaft and when the torque acting on the shaft and / or impeller increases and / or changes more rapidly.

[0005] The so-called Hirth coupling, in which both the end face of the shaft and the opposing end face of the impeller are provided with complementary intermeshing tapered teeth, is a proven solution for realizing such a connection between impeller and shaft, and by its self-aligning nature, it counteracts the above-mentioned relative radial or torsional displacements. However, the manufacture of the required tooth profiles on the end faces of the impeller and shaft is laborious, time-consuming and, as a result, expensive.

[0006] DE 10 2010040288 A1 seeks to solve this problem by arranging one or two intermediate elements between the end face of the shaft of the turbomachine and the opposite end face of the impeller, which are arranged concentrically around the bolt connecting the shaft and the impeller. One end face of the intermediate element is fixed by a bolt to the end face of the shaft or the impeller, and the other end face is provided with a toothed profile. Thus, by using two complementary spacers, one attached to the shaft end face and the other to the opposite impeller face, a hearth coupling between the shaft and the impeller can be realized without the difficulties of providing gears on the end faces of the shaft and the impeller.

[0007] However, in the system of DE 10 2010040288 A1, bolted connections must be made between one or more intermediate elements and the shaft and / or impeller, which involves an extra step in the manufacturing process and therefore a higher cost price. Furthermore, these bolted connections must consequently remain aligned during machine operation, thus changing the problem of radial relative displacement between two axially connected rotating members from a connection between the impeller and the shaft to a connection between one or more spacers and the shaft and / or impeller. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] German publication number 102010040288 Summary of the Invention [Problem to be solved by the invention]

[0009] Therefore, another solution is needed to prevent relative radial displacement between the shaft and impeller of a turbomachine. [Means for solving the problem]

[0010] The object of the present invention is to provide a solution to at least one of the above mentioned drawbacks.

[0011] According to a first aspect of the present invention, a turbomachine is disclosed comprising a shaft having a first end and an impeller disposed on and coupled to the first end of the shaft, the turbomachine comprising a friction ring clamped between an axial surface (end face) of the first end of the shaft and an opposing axial surface (end face) of the impeller, the friction ring comprising a friction ring clamped between an axial surface (end face) of the first end of the shaft and an opposing axial surface (end face) of the impeller.

[0012] In some embodiments, the turbomachine is characterized in that the impeller is coupled to the first end of the shaft by a draw bolt or a draw bar connection.

[0013] In a draw bolt or draw bar connection, the shaft and the impeller are provided with the facility to receive a draw bolt or draw bar. The draw bolt or draw bar is fixedly attached to one of the two parts, for example by screwing a first end of the draw bolt or draw bar into a thread provided for this purpose on one of the two parts. The bolt or bar is then biased by applying a tensile force between its two ends. The second part, which receives the bolt or bar but is not yet fixedly attached, is then clamped between the first part and a closure element for the connection. This closure element can be, for example, a nut that is screwed onto the second end of the bolt or bar. As soon as the closure element has been fixed in the desired way, for example by tightening the nut with the desired torque, the tensile force is released.

[0014] It is an advantage of these embodiments that the drawbolt or drawbar connection is a simple and inexpensive connection technique.

[0015] In some embodiments, the impeller is coupled to the first end of the shaft by a draw bolt connection, and the turbomachine is characterized in that the friction ring is clamped between an axial surface (end face) of the first end of the shaft and an opposing axial surface (end face) of the impeller by the draw bolt connection.

[0016] It is an advantage of these embodiments that no additional parts are required to clamp the friction ring.

[0017] In some embodiments, the turbomachine is characterized in that the impeller and the shaft are made of substantially different materials, the impeller material having a lower mass density than the shaft material.

[0018] It is an advantage of these embodiments that the low mass density of the impeller allows for good impeller dynamics. For example, the impeller may consist essentially of aluminium and the shaft may consist essentially of steel.

[0019] In some embodiments, the turbomachine is characterized in that the friction ring is adapted to counteract relative displacement between an axial surface (end face) of the first end of the shaft and an opposing axial surface (end face) of the impeller.

[0020] Generally, the centers of mass of the rotating components of a turbomachine are not all exactly coincident with the axis of rotation of the machine. During operation of the turbomachine, shock loads acting on the centers of mass of the rotating components result in a shear force between the end face of the first end of the shaft and the opposing end face of the impeller. This shear force is countered by static and kinetic friction forces between said end faces. If the shear force exceeds the static friction force, a relative displacement may occur between said end faces. Such a relative displacement may increase the imbalance of the machine to an unacceptable level.

[0021] It is an advantage of these embodiments that the turbomachine does not require a coupling between the impeller and the shaft that is self-aligning due to its geometry, such as a Hirth coupling, the coupling between the impeller and the shaft acquires a greater tendency to self-align due to the presence of the friction ring, and preferably the friction ring is so thin and light relative to the impeller and the shaft that the degree of alignment quality of the turbomachine is not adversely affected even if the friction ring itself is not centered relative to the rotation axis of the machine.

[0022] It is a further advantage of these embodiments that the turbomachine can operate at a higher speed, a higher power, a higher mass flow rate, or a combination of two or more of the above, compared to a turbomachine without a friction ring for a comparable alignment quality of the turbomachine.

[0023] It is a further advantage of these embodiments that the turbomachine can change speed more quickly for the same allowable vibration level compared to a turbomachine without a friction ring.

[0024] It is a further advantage of these embodiments that the ratio of impeller mass to shaft mass can be greater than in turbomachines without friction rings, for the same allowable vibration level.

[0025] It is a further advantage of these embodiments that the turbomachine shaft can have a smaller diameter and / or a smaller mass for the same allowable vibration level than a turbomachine shaft without a friction ring.

[0026] A further advantage of these embodiments of the turbomachine is that, for the same shaft diameter or the same contact pressure on the end faces of the shaft and the impeller, a greater torque can be transmitted between the shaft and the impeller compared to a turbomachine without a friction ring.

[0027] In some embodiments, the friction ring is adapted to counteract relative displacement between an axial surface (end face) of the first end of the shaft and an opposing axial surface (end face) of the impeller, and the turbomachine is characterized in that the axial surface of the friction ring is at least partially covered by a coating, the coating comprising a substantially uniform layer of a metal or metal alloy as well as discrete particles at least partially surrounded by the layer of metal or metal alloy, the median diameter of the particles being greater than a thickness of the layer of metal or metal alloy.

[0028] In some embodiments, the axial surface of the friction ring is at least partially covered by a coating, the coating including a substantially uniform layer of a metal or metal alloy as well as discrete particles at least partially surrounded by the layer of metal or metal alloy, the median diameter of the particles being greater than the thickness of the layer of metal or metal alloy, and the turbomachine is characterized in that the hardness of the particles is greater than the hardness of a material of the opposing axial surface (end face) of the impeller.

[0029] The advantage of these embodiments is that the particles on the friction ring can penetrate into the axial face of the impeller, thereby increasing the coefficient of friction between the impeller and the friction ring. This allows, for example, an aluminum impeller to be used in combination with a steel shaft. This combination would be difficult to achieve without the presence of the friction ring, since the permissible surface pressure of the aluminum or the coefficient of friction between the two surfaces, steel and aluminum, respectively, would be too low to sufficiently counteract the unacceptable displacements between the impeller and the end face of the shaft during normal operation of the turbomachine.

[0030] In some embodiments, the hardness of the particles in the coating of the friction ring is greater than the hardness of the material of the opposing axial surface (end face) of the impeller, and the turbomachine is characterized in that the particles consist essentially of diamond or ceramic material.

[0031] In some embodiments, the turbomachine is characterized in that the impeller is a centrifugal impeller.

[0032] In some embodiments, the turbomachine is characterized as being a compressor, a blower, or a turbine.

[0033] According to a second aspect of the present invention, a method for countering relative displacement between an impeller of a turbomachine and a shaft of the turbomachine, the impeller being coupled to a first end of the shaft, both arranged to move together in rotation about an axis of rotation, the method comprising clamping a friction ring between an axial surface of the first end of the shaft and an opposing axial surface of the impeller.

[0034] The technical effects and advantages of the method according to the invention correspond to those of the turbomachine according to the invention.

[0035] According to a third aspect of the invention, the use of a friction ring in a turbomachine comprising a shaft having a first end and an impeller disposed on and coupled to the first end of the shaft, arranged together to rotate about an axis of rotation, the friction ring being clamped between an axial surface of the first end of the shaft and an opposing axial surface of the impeller so as to counteract relative displacement between the axial surface of the first end of the shaft and the opposing axial surface of the impeller.

[0036] The technical effects and advantages of using the friction ring according to the invention are in accordance with the technical effects and advantages of the turbomachine according to the invention. [Brief description of the drawings]

[0037] [Figure 1] 1 is a schematic exploded view of one embodiment of a turbomachine in accordance with the present invention; [Diagram 2] 2 shows a schematic cross-section along axis 1 of FIG. [Diagram 3] 3 is a schematic enlarged view of region X in the cross section of FIG. 2. FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] The present invention is described below with reference to specific embodiments. These specific embodiments are representative of the present invention, but should not be considered as limiting. The present invention is not limited to the specific embodiments shown and / or described below, and alternative embodiments can occur within the spirit of the present specification. The figures are merely schematic, do not necessarily represent all parts of the present invention, are not limiting, and are not necessarily to scale.

[0039] When reference is made herein to "one embodiment," this means that a particular feature, structure, or characteristic described with reference to that particular embodiment is included in one or more embodiments of the present invention. References herein to "one embodiment," "some embodiments," "preferred embodiments," etc. may, but do not necessarily, refer to the same embodiment. Furthermore, particular features, structures, or characteristics of a particular embodiment may be combined in any suitable manner in one or more embodiments.

[0040] Figure 1 is a schematic exploded view of one embodiment of a turbomachine according to the present invention, although not all parts of the turbomachine are visible in Figure 1.

[0041] The illustrated embodiment comprises a shaft 10, an impeller 20, a friction ring 30, a draw bolt 40 and a nut 50. Preferably, each of these parts is substantially axisymmetric, with deviations from axisymmetric resulting from tolerance limitations of the manufacturing process used, wear or the addition of one or more structures, the total mass of which is less than 10%, preferably less than 1%, more preferably less than 0.1%, even more preferably less than 0.01% and most preferably less than 0.001% of the mass of the part to which these one or more structures are added. Preferably, the alignment of each of these parts complies with the ISO 21940 standard at grade G1, more preferably at grade G0.7.

[0042] Preferably, these parts are aligned relative to one another such that their respective axes of symmetry substantially coincide. These parts are arranged to rotate together about the axis of rotation 1 of the turbomachine. Preferably, these parts are aligned such that their respective axes of symmetry substantially coincide with the axis of rotation 1.

[0043] Figures 2 and 3 show these components in more detail. Below, the components and assemblies of the illustrated embodiment will be described, followed by a description of how they cooperate in the illustrated embodiment.

[0044] The shaft 10 comprises a first axial bore 11 that is substantially concentric with an axis of symmetry of the shaft 10. The shaft 10 comprises a second axial bore 12 that is substantially concentric with the first axial bore 11 and with an axis of symmetry of the shaft 10. The first axial bore 11 is deeper than the second axial bore 12. The first axial bore 11 has a smaller diameter than the second axial bore 12.

[0045] Preferably, the second axial bore 12 has a depth greater than 1 mm, more preferably greater than 3 mm, even more preferably greater than 5 mm, and most preferably greater than 10 mm. Preferably, the second axial bore 12 has a depth less than 50 mm, more preferably less than 40 mm, even more preferably less than 30 mm, and most preferably less than 20 mm.

[0046] Preferably, the first axial bore 11 has a depth at least 20 mm greater than the depth of the second axial bore 12, more preferably at least 30 mm greater, even more preferably at least 40 mm greater, and most preferably at least 50 mm greater. Preferably, the first axial bore 11 has a depth less than 100 mm, more preferably less than 90 mm, even more preferably less than 80 mm, and most preferably less than 70 mm, greater than the depth of the second axial bore 12.

[0047] Preferably, the second axial bore 12 has a diameter greater than 60% of the diameter of the shaft 10, more preferably greater than 65% of the diameter of the shaft, even more preferably greater than 70% of the diameter of the shaft, and most preferably greater than 75% of the diameter of the shaft. Preferably, the second axial bore 12 has a diameter less than 95% of the diameter of the shaft 10, more preferably less than 90% of the diameter of the shaft, even more preferably less than 85% of the diameter of the shaft, and most preferably less than 80% of the diameter of the shaft.

[0048] Preferably, the first axial bore 11 has a diameter greater than 10% of the diameter of the shaft 10, more preferably greater than 15% of the diameter of the shaft, even more preferably greater than 20% of the diameter of the shaft, and most preferably greater than 25% of the diameter of the shaft. Preferably, the first axial bore 11 has a diameter smaller than 50% of the diameter of the shaft 10, more preferably smaller than 45% of the diameter of the shaft, even more preferably smaller than 40% of the diameter of the shaft, and most preferably smaller than 35% of the diameter of the shaft.

[0049] The first axial bore 11 and the second axial bore 12 define two substantially concentric annular end faces on the shaft 10. The first end face 13 is located at the end of the shaft and is defined by the diameter of the shaft and the diameter of the second axial bore 12. The second end face 14 is recessed into the second axial bore 12 and is defined by the diameters of the first and second axial bores. Preferably, both end faces are substantially planar except for radiuses or chamfers at their edges and lie in a plane substantially perpendicular to the axis of symmetry of the shaft 10.

[0050] The shaft 10 is preferably made of steel, which may be unalloyed, low alloyed, or highly alloyed, and may be made of other suitable materials known to those skilled in the art.

[0051] The illustrated embodiment comprises a centrifugal impeller 20. Such an impeller can be used, for example, in a centrifugal compressor, a blower or a turbine. At its axial end facing the shaft 10, the impeller 20 is substantially cylindrical and comprises an axial projection 22 that is substantially concentric with the axis of symmetry of the impeller 20. The impeller 20 and the axial projection 22 comprise an axial bore 21 that is substantially concentric with the axis of symmetry of the impeller 20 and the axial projection 22 and that extends over the entire axial depth of the impeller 20 and the axial projection 22.

[0052] The axial projection 22 defines two end faces at the axial end of the impeller 20. The inner diameter of the first end face 23 is defined by the outer diameter of the axial projection 22. The outer diameter of the first end face 23 is defined by a diameter at which the axial end is no longer substantially flat and lies in a plane substantially perpendicular to the axis of symmetry of the impeller 20. The second end face 24 is located at the axial end of the axial projection 22. The inner diameter of the second end face 24 is defined by the diameter of the axial bore 21. The outer diameter of the second end face 24 is defined by the outer diameter of the axial projection 22.

[0053] Preferably, the first end face 23 of the impeller 20 has an outer diameter that is greater than the outer diameter of the first end face 13 of the shaft 10 .

[0054] The axial projection 22 has an outer diameter smaller than the inner diameter of the second axial bore 12 of the shaft 10. This has the advantage that the axial projection 22 can be inserted into the second axial bore 12 during assembly of the turbomachine. Preferably, the outer diameter of the axial projection 22 is at most 10% smaller than the inner diameter of the second axial bore 12 of the shaft 10, more preferably at most 1% smaller and most preferably at most 0.1% smaller. This small difference in diameter has the advantage that the outer wall of the axial projection 22 comes into contact with the inner wall of the second axial bore 12 during assembly of the turbomachine, thereby contributing to the relative alignment of the impeller 20 to the shaft 10. Preferably, the outer diameter of the axial projection 22 is at least 0.01% smaller than the inner diameter of the second axial bore 12.

[0055] Preferably, the length of the axial protrusion 22 is shorter than the depth of the second axial bore 12 of the shaft 10. This has the advantage that the second end faces 14 and 24 of the shaft 10 and the impeller 20, respectively, cannot come into contact when the turbomachine is assembled. Preferably, the length of the axial protrusion 22 is at least 5% shorter than the depth of the second axial bore 12 of the shaft 10. However, in some not shown, non-preferred embodiments, the length of the axial protrusion 22 can be equal to or longer than the depth of the second axial bore 12, and the end faces 14 and 24 can come into contact when the turbomachine is assembled.

[0056] In some embodiments, the impeller 20 can be made substantially of steel. The steel can be unalloyed, low alloyed, or highly alloyed. In some embodiments, the impeller 20 can be made substantially of aluminum or an aluminum alloy. In some embodiments, the impeller 20 can consist substantially of a metal or an alloy of a metal other than steel or aluminum, preferably having a mass density lower than steel.

[0057] In some embodiments, the impeller 20 may be substantially constructed of a different material than the shaft 10, and the hardness of both materials need not necessarily be the same. A preferred embodiment includes an impeller 20 constructed substantially of aluminum or an aluminum alloy, and a shaft 10 constructed substantially of steel.

[0058] The friction ring 30 consists of a substantially circular, substantially planar disk with concentric inner and outer diameters. Preferably, the friction ring 30 has an inner diameter larger than the outer diameter of the axial projection 22. This has the advantage that the friction ring 30 can be placed around the axial projection 22 during assembly of the turbomachine. Preferably, the inner diameter of the friction ring 30 is at most 10% larger than the outer diameter of the axial projection 22, more preferably at most 1% larger and most preferably at most 0.05% larger. This small difference in diameter has the advantage that the outer wall of the axial projection 22 comes into contact with the inner wall of the friction ring 30 during assembly of the turbomachine, thereby contributing to the relative alignment of the friction ring 30 to the impeller 20. Preferably, the inner diameter of the friction ring 30 is at least 0.01% larger than the outer diameter of the axial projection 22. In some embodiments not shown, the friction ring 30 can include an axial projection on its outer diameter. This projection can, for example, serve to visually confirm the presence of the friction ring. In some embodiments not shown, friction ring 30 can take a shape other than a substantially circular, substantially planar disk having concentric inner and outer diameters.

[0059] Both end faces 33a, 33b of the friction ring 30 can be substantially covered with a coating. Preferably, at least 50% of both end faces are covered with the coating, more preferably at least 60%, even more preferably at least 70%, and most preferably both end faces are substantially completely covered. The coating comprises a substantially uniform layer of metal or metal alloy as well as discrete particles partially surrounded by the layer of metal or metal alloy. The median diameter of the discrete particles is greater than the median diameter of the thickness of the uniform layer and the average diameter of the discrete particles is greater than the average thickness of the uniform layer. The discrete particles are substantially made of diamond or ceramic material. The hardness of the discrete particles is greater than the hardness of steel. Preferably, the discrete particles have an average diameter greater than 10 microns, more preferably greater than 20 microns, even more preferably greater than 30 microns, and most preferably greater than 40 microns. Preferably, the average thickness of the uniform layer is greater than 60% of the average thickness of the discrete particles. Preferably, the average thickness of the uniform layer is less than 90% of the average thickness of the discrete particles. Preferably, the surface density of the discrete particles is 50 particles / mm 2 Larger, preferably 100 particles / mm 2 Larger, preferably 200 particles / mm 2 Larger, most preferably 500 particles / mm 2 Preferably, the discrete particles are substantially uniformly distributed on the end faces 33a and 33b so as not to degrade the alignment quality of the friction rings.

[0060] In the illustrated embodiment, the impeller 20 is overhangingly coupled to one end of the shaft 10 by inserting a draw bolt 40 into the axial bores 11 and 21 of the shaft 10 and the impeller 20, respectively. The impeller 20 is thereby oriented with respect to the axis 10 with the axial projection 22 pointing towards the axis 10. Preferably, the friction ring 30 is disposed around the axial projection 22 so as to be located between the end faces 13 and 23. However, in some non-illustrated embodiments, the friction ring 30 may be disposed in the second axial bore 12 so as to be located between the end faces 14 and 24.

[0061] The draw bolt is threaded at both ends 41 a and 41 b. Preferably, the draw bolt comprises a neck portion 43 having a diameter smaller than the diameter of the axial bore 21. Preferably, the draw bolt comprises a shoulder portion 42. Preferably, the diameter of the shoulder portion 42 is substantially the same as the diameter of the axial bore 21. Preferably, prior to insertion, the draw bolt 40 is subjected to a treatment, such as cooling with liquid nitrogen, so that the shoulder portion 42 can be inserted and positioned within the axial bore 21.

[0062] The threaded end 41b is screwed into the first axial bore 11 of the shaft 10 with a matching thread. A nut 50 with a matching thread is screwed onto the end 41a. By tightening the nut 50, the impeller 20 and the shaft 10 move towards each other. The axial projection 22 is inserted into the second axial bore 12.

[0063] The nut 50 is tightened at least until the first end faces 13 and 23 of the shaft 10 and the impeller 20, respectively, contact both end faces 33a and 33b of the friction ring 30, respectively, or, in some non-illustrated non-preferred embodiments, until the second end faces 14 and 24 of the shaft 10 and the impeller 20, respectively, contact the second end faces 33a and 33b of the friction ring 30, respectively. Preferably, the nut 50 in the final position contacts the third end face 25 of the impeller 20. Preferably, the draw bolts 40 are pretensioned before the nut 50 is tightened. Preferably, the tension of the draw bolts occurs at a tension of 200 kN or more. Preferably, the tension of the draw bolts occurs at a tension of 350 kN or less. Preferably, the nut 50 is tightened until the first axial end faces 13 and 23 are pressed against the respective end faces 33a and 33b with a contact pressure of 500 MPa or more. Preferably, the nut 50 is tightened until the first axial end faces 13 and 23 are pressed against the respective end faces 33a and 33b with a contact pressure of 800 MPa or less. By energizing the draw bolt 40 and tightening the nut 50, the discrete particles on the friction ring end faces 33a and 33b are forced into the respective end faces 13 and 23 of the shaft 10 and impeller 20. Preferably, the discrete particles penetrate into the end faces 13 and 23 by more than 2 microns, more preferably by more than 5 microns, and most preferably by more than 10 microns.

[0064] The impeller 20 is connected to the shaft 10 in an overhanging manner so that there are bearings (not shown) only along one axial side of the impeller 20. Preferably, the impeller 20 of the turbomachine rotates at a speed of more than 5000 revolutions per minute about the axis of rotation 1 at normal operating points of the machine, more preferably more than 10000 revolutions per minute, most preferably more than 20000 revolutions per minute. Preferably, the mass of the impeller 20 is 50% or more of the mass of the shaft 10. Preferably, the mass of the impeller 20 is 400% or less of the mass of the shaft 10.

[0065] The single-sided bearing, together with the speeds achieved and the mass ratio between impeller and shaft, places very high demands on the alignment of the turbomachine. Preferably, the alignment of the turbomachine for rotation about the axis of rotation 1 after assembly complies with the ISO 21940 standard grade G6.3, more preferably grade G2.5, most preferably grade G1.

[0066] During operation of the turbomachine, centrifugal forces acting on the centers of mass of the rotating components, which are generally not all exactly coincident with the axis of rotation 1, result in shear forces between the end face 33a of the friction ring 30 and the end face 13 of the shaft 10, and between the end face 33b of the friction ring 30 and the end face 23 of the impeller 20. These shear forces are countered by static and dynamic friction forces between the end faces. If the shear forces exceed the static friction forces, radial displacements can occur between the end faces. Such radial displacements can increase the imbalance of the machine to unacceptable levels.

[0067] Due to the presence of the friction ring 30, the discrete particles contained in the coating on the end faces 33a and 33b are pressed into the end faces 13 and 23 during assembly of the turbomachine as described above. This pressing results in an increase in the coefficient of friction, and therefore the frictional force, between the end face 33a of the friction ring 30 and the end face 13 of the shaft 10, and between the end face 33b of the friction ring 30 and the end face 23 of the impeller 20. Preferably, the static coefficient of friction between the end faces is greater than 0.4, more preferably greater than 0.5, even more preferably greater than 0.6 and most preferably greater than 0.7.

[0068] Due to the larger coefficient of friction and the resulting larger frictional force between the opposing end faces, radial displacement between the end faces is counteracted and reduced in amplitude or avoided.

[0069] An advantage of the embodiments of the turbomachine according to the invention is that the impeller can be made of aluminum. An aluminum impeller is lighter, allows a more dynamic rotor behavior and does not require a more precise alignment, but without a friction ring, the friction forces between the aluminum impeller and the steel shaft are too small to counteract the radial displacement between the impeller and the shaft under normal operating conditions of the turbomachine. Those skilled in the art will understand that other advantages of the mentioned embodiments also apply to any combination of shaft and impeller materials, such as, for example, a steel impeller and a steel shaft.

[0070] It is an advantage of one embodiment of the turbomachine according to the invention that no self-aligning coupling, such as a Hirth coupling, is required between the impeller 20 and the shaft 10 .

[0071] It is an advantage of an embodiment of a turbomachine according to the invention that it can operate at higher speeds than a turbomachine without friction rings for the same allowable vibration level.

[0072] It is an advantage of an embodiment of the turbomachine according to the invention that, for the same allowable vibration level, it is possible to change speed more quickly than a turbomachine without friction rings.

[0073] It is an advantage of an embodiment of the turbomachine according to the invention that, for the same allowable vibration level, the ratio between the mass of the impeller and the mass of the shaft can be greater than in a turbomachine without friction rings. [Explanation of symbols]

[0074] 1 Rotation axis 10 Shaft 13 Axial surface 20 Impeller 23 Axial surface 30 Friction Ring

Claims

1. 1. A turbomachine comprising: a shaft (10) having a first end, and an impeller (20) disposed on and coupled to the first end of the shaft, the impeller (20) being arranged together to rotate about an axis of rotation (1), The shaft (10) comprises a first axial bore (11) and a second axial bore (12) at the first end, the first axial bore (11) being deeper than the second axial bore (12), with a stepped axial bore; the impeller (20) comprises a stepped axial protrusion including a first axial protrusion (21) and a second axial protrusion (22), the first axial bore (11) being arranged to receive the first axial protrusion (21) and the second axial bore (12) being arranged to receive the second axial protrusion (22); The turbomachine further comprises a friction ring (30) clamped between an axial surface (13) of the first end of the shaft (10) surrounding the second axial bore (12) and an opposing axial surface (23) of the impeller (20) surrounding the second axial projection (22); an axial surface (33a, 33b) of said friction ring (30) being at least partially covered with a coating, said coating comprising a substantially uniform layer of a metal or metal alloy as well as discrete particles at least partially surrounded by said layer of metal or metal alloy; 10. A turbomachine, comprising: an impeller (20) coupled to the first end of the shaft (10) by a draw bolt connection.

2. The turbomachine of claim 1, wherein the impeller (20) and the shaft (10) are made of substantially different materials, the material of the impeller (20) having a lower mass density than the material of the shaft (10).

3. 3. The turbomachine of claim 1 or 2, wherein the friction ring (30) is adapted to counteract relative displacement between the axial surface (13) of the first end of the shaft (10) and the opposing axial surface (23) of the impeller (20).

4. The turbomachine of claim 3, wherein the median diameter of the discrete particles is greater than a thickness of the layer of the metal or metal alloy.

5. The turbomachine of claim 4, wherein the hardness of the discrete particles is greater than the hardness of the material of the opposing axial surface (23) of the impeller (20).

6. The turbomachine of claim 5 , wherein the discrete particles consist essentially of diamond or ceramic material.

7. The turbomachine of any one of claims 1 to 6, wherein the impeller (20) is a centrifugal impeller.

8. The turbomachine of claim 1 , wherein the turbomachine is a compressor, a blower, or a turbine.

9. 1. A method for counteracting relative displacement between a turbomachine impeller (20) and a turbomachine shaft (10), the impeller (20) being coupled to a first end of the shaft, the impeller (20) and the shaft being arranged to move together in rotation about an axis of rotation (1), the method comprising: inserting a first axial protrusion (21) and a second axial protrusion (22) of the impeller (20) into a first axial bore (11) and a second axial bore (12) of the first end of the shaft (10), the first axial bore (11) being deeper than the second axial bore (12), the first axial bore (11) being arranged to receive the first axial protrusion (21), and the second axial bore (12) being arranged to receive the second axial protrusion (22); clamping a friction ring (30) between an axial surface (13) of the first end of the shaft (10) surrounding the second axial bore (12) and an opposing axial surface (23) of the impeller (20) surrounding the second axial projection (22); coupling the impeller (20) to the first end of the shaft (10) by a draw bolt connection; Including, The method of claim 1, wherein an axial surface (33a, 33b) of the friction ring (30) is at least partially covered by a coating, the coating comprising a substantially uniform layer of a metal or metal alloy as well as discrete particles at least partially surrounded by the layer of metal or metal alloy.

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