METHOD FOR MACHINING RIBS OR GROOVES FOR AIR OR GAS BEARINGS OF A ROTARY-SHAFT-COMPRISING WORKPIECE OF A COMPRESSOR, AND METHOD FOR ASSEMBLING THE COMPONENTS OF THE WORKPIECE

Abstract

A method for machining ribs or grooves on a workpiece including a shaft (7) and an air or gas axial bearing (24) attached to the shaft or forming a part of the shaft. The workpiece is rotated about a longitudinal axis of a centrifugal compressor. All of the ribs or grooves are obtained at once by the machining tool on a workpiece portion driven such that it rotates, by moving the workpiece or the tool holder in a longitudinal direction, the machining tool moving back and forth with a machining position in contact with the workpiece and a position wherein it is not in contact with the workpiece from the beginning to the end of the workpiece portion. Reciprocating motions of the machining tool are synchronised with the sinusoidal program, as well as with the desired, programmed arrangement of the ribs or grooves to be produced on the workpiece portion.

Claims

1. A method for machining ribs or grooves on a workpiece intended to be rotated about a longitudinal axis (A-A) of a centrifugal compressor (1), the workpiece comprising a shaft (7), on which or in which a rotor structure (16) with at least one permanent magnet (16a) of an electric motor is mounted or attached in order to drive the shaft such that it rotates, and/or an air or gas axial bearing (24) attached to one end of the shaft (7) or forming a part of the shaft (7), the centrifugal compressor (1) further comprising a casing (2) with a fluid inlet (5) and a compressed fluid outlet (6), a first compressor wheel (8) of the workpiece and a second compressor wheel (10) of the workpiece mounted at two ends of the shaft (7) in the casing (2), a front air or gas radial bearing (18) mounted on a first end of the shaft (7), and a rear air or gas radial bearing (22) mounted on a second end of the shaft (7), the machining method being carried out in a machining unit (100) adapted to receive the workpiece and comprising a tool holder (110) with a tool for machining the ribs or grooves on at least one portion of the workpiece, the method being characterised in that all of the ribs or grooves (24a; 32) are obtained at once by the machining tool on a workpiece portion of the shaft (7) rotatably driven, by moving the shaft (7) of the workpiece or the tool holder (110) in a longitudinal machining direction, said machining tool moving back and forth with a machining position in contact with the workpiece and a position wherein it is not in contact with the workpiece from the beginning to the end of the workpiece portion, in that the reciprocating motions of the machining tool are synchronised with the sinusoidal program set up in the machining unit, as well as with the desired, programmed arrangement of the ribs or grooves to be produced on the workpiece portion, in that all of the ribs or grooves (32) are obtained at once on a first workpiece portion of the first end of the shaft (7) from the beginning of the first portion to the end of the first portion, by the reciprocating motions of the machining tool and the displacement in a longitudinal machining direction of the rotating shaft (7) according to how the machining unit (100) was programmed, and in that all of the ribs or grooves (32) are obtained at once on a second workpiece portion of the second end of the shaft (7) from the beginning of the second portion to the end of the second portion, by the reciprocating motions of the machining tool and the displacement in a longitudinal machining direction of the rotating shaft (7) or tool holder (110).

2. The machining method according to claim 1, wherein the workpiece is the shaft (7), which is made of tungsten carbide or ceramic, and wherein the head of the machining tool (120) is made of diamond for machining the ribs or grooves on the first machined portion and on the second machined portion of the rotating shaft (7).

3. The machining method according to claim 2, characterised in that during the machining of the ribs or grooves (32), a change of orientation of the ribs or grooves (32) occurs when passing the halfway point along the first workpiece portion of the first end of the shaft (7) according to how the machining unit was programmed so as to obtain V-shaped grooves over the length of the first machined portion in order to generate an air or gas pressure in the front radial bearing (18) disposed on the first portion during operation of the compressor with the rotation of the shaft (7) above a limit speed so as to no longer have any mechanical contact with the front radial bearing (18).

4. The machining method according to claim 2, characterised in that during the machining of the ribs or grooves (32), a change of orientation of the ribs or grooves (32) occurs when passing the halfway point along the second workpiece portion of the second end of the shaft (7) according to how the machining unit (100) was programmed so as to obtain V-shaped grooves over the length of the second machined portion in order to generate an air or gas pressure in the rear radial bearing (22) disposed on the second portion during operation of the compressor (1) with the rotation of the shaft (7) above a limit speed so as to no longer have any mechanical contact with the rear radial bearing (22).

5. The machining method according to claim 1, wherein the workpiece is the air or gas axial bearing (24) in the form of a disc, characterised in that the ribs or grooves (24a) and the arrangement thereof are programmed in the machining unit (100) to activate the machining tool (120) so as to produce all of the ribs or grooves at once on a first face of the disc by moving the machining tool, or the rotating disc in a single machining direction from the periphery of the disc to the bottom of the annular area of the ribs or grooves or conversely, and by the reciprocating motions of the machining tool (120) according to how the machining unit (100) was programmed.

6. The machining method according to claim 5, characterised in that all of the ribs or grooves (24a) are produced at once on a second face of the disc by moving the machining tool, or the rotating disc in a single machining direction from the periphery of the disc to the bottom of the annular area of the ribs or grooves (24a) or conversely, and by the reciprocating motions of the machining tool (120) according to how the machining unit (100) was programmed.

7. The machining method according to claim 6, characterised in that all of the ribs or grooves (24a) are produced in the form of spirals with the same orientation on the two faces or with two different orientations so as to generate films of air via the grooves when rotating the shaft to hold the axis in a position that is longitudinally well centred during operation of the centrifugal compressor (1).

8. The machining method according to claim 7, characterised in that all of the ribs or grooves (24a) on one face or the two faces of the disc are machined before or after the machining of the ribs or grooves on the first portion and/or the second portion of the shaft (7).

9. A method for machining the ribs or grooves on a workpiece (7, 24) according to claim 1, and for assembling the components of the workpiece, said components being the shaft (7) on which is attached the air or gas axial bearing (24) in the form of a disc attached to one end of the shaft (7), and on which or in which the rotor structure (16), having at least one permanent magnet (16a), of an electric motor is disposed or attached between the first and second portions of the shaft (7), characterised in that all of the ribs or grooves (24a) on one face or the two faces of the disc are machined after the machining of the ribs or grooves on the first portion and/or the second portion of the shaft (7) and before the placing or attachment of the rotor structure (16) having at least one permanent magnet (16a) on or in the shaft between the first portion and the second portion, which are machined from the shaft (7).

10. The method for machining the ribs or grooves on a workpiece (7, 24) according to claim 1, and for assembling the components of the workpiece, said components being the shaft (7) on which is attached the air or gas axial bearing (24) in the form of a disc attached to one end of the shaft (7), and on which or in which the rotor structure (16), having at least one permanent magnet (16a), of an electric motor is disposed or attached between the first and second portions of the shaft (7), characterised in that the rotor structure (16), having at least one permanent magnet (16a), of an electric motor is disposed or attached between the first and second portions of the shaft (7) before the ribs or grooves are machined on the first portion and the second portion of the shaft (7), and in that all of the ribs or grooves (24a) on one face or the two faces of the disc are machined after the machining of the ribs or grooves on the first portion and/or the second portion of the shaft (7).

11. A workpiece intended to be rotated about a longitudinal axis (A-A) of a centrifugal compressor (1) and intended to have ribs or grooves machined according to the machining method according to claim 1, characterised in that the workpiece comprises at least one shaft (7), on which or in which a rotor structure (16), having at least one permanent magnet (16a), of an electric motor is mounted or attached in order to drive the shaft such that it rotates, and/or an air or gas axial bearing (24) attached to one end of the shaft (7) or forming a part of the shaft (7).

12. The workpiece according to claim 11, characterised in that the permanent magnet rotor structure (16) is disposed on an outer surface of the shaft between the first portion and the second portion, machined with ribs or grooves, of the shaft (7).

13. The workpiece according to claim 12, characterised in that the rotor structure (16) comprises one or more permanent magnets (16a) secured to the shaft (7) and covered by a lining (16b), and in that two flanges (16c) or one flange (16c) and a shoulder on the shaft (7) are attached or placed at the lateral ends of the lining (16b) to allow the rotor (16) to withstand the centrifugal forces at high speeds.

14. The workpiece according to claim 13, characterised in that the flanges (16c) are bonded or shrunk between the shaft (7) and the lining (16b).

15. The workpiece according to claim 11, characterised in that the permanent magnet rotor structure (16) is disposed inside the shaft (7) in tubular form, and in that the one or more permanent magnets (16a) are shrunk inside the shaft (7) and disposed on a rod (11) or attached to two end rods (11b, 11d).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The purposes, advantages and features of the present invention will be better understood upon reading the following detailed description of one embodiment of the invention, which is given as a non-limiting example and illustrated in the accompanying drawings, in which:

[0026] FIG. 1 shows a longitudinal sectional view, along the axis A-A, of a high-speed centrifugal compressor according to the invention,

[0027] FIGS. 2a and 2b show a longitudinal sectional view, along the axis A-A, of the shaft with the compressor wheels, the aerodynamic axial and radial bearings and the rotor structure that has one or more permanent magnets according to the invention, and a plan view from the side of the first compressor wheel and the axial bearing according to the invention,

[0028] FIG. 3 shows a graph of the difference between a radial displacement of the outer surface of the shaft and that of the inner surface of the one or more permanent magnets according to the invention,

[0029] FIG. 4 is a three-dimensional view of the assembly in FIGS. 2a and 2b showing the grooves or ribs in each static radial bearing shown raised above the shaft according to the invention,

[0030] FIG. 5 shows a first alternative embodiment of a longitudinal sectional view, along the axis A-A, of the shaft with the compressor wheels, at least the axial bearing integral with the shaft and the rotor structure with one or more permanent magnets according to the invention disposed inside the shaft,

[0031] FIG. 6 shows a second alternative embodiment of a longitudinal sectional view, along the axis A-A, of the shaft with the compressor wheels, at least the axial bearing integral with the shaft and the rotor structure with one or more permanent magnets according to the invention disposed inside the shaft,

[0032] FIG. 7 is a plan view of the air or gas axial bearing,

[0033] FIG. 8 is a view of the ribs or grooves disposed at one end of the shaft on which is disposed the static radial bearing shown via a longitudinal section according to the invention,

[0034] FIG. 9 is a view of the shaft with grooves or ribs disposed at the two ends of the shaft and on which are respectively disposed the static air or gas radial bearings according to the invention,

[0035] FIG. 10 is an overview of a machining unit for machining the grooves or ribs on the shaft of the centrifugal compressor according to the invention,

[0036] FIG. 11 is a graph of a precision rib produced on the shaft for the air or gas radial bearings according to the invention, and

[0037] FIG. 12 is a three-dimensional overview of a machining unit for machining the grooves or ribs on an air or gas axial bearing of the centrifugal compressor according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0038] In the present description, all the components forming part of the centrifugal compressor, which are well known in the prior art, are only briefly described herein, as the invention essentially relates to the manner in which ribs or grooves are produced on two portions of a shaft in order to be covered by two static air or gas radial bearings respectively, or on an air or gas axial bearing.

[0039] FIG. 1 shows a cross-section, along the longitudinal axis A-A, of a high-speed centrifugal compressor 1. The centrifugal compressor comprises, in a casing 2, a shaft 7 made of tungsten carbide or ceramic, mounted such that it rotates about a longitudinal axis A-A passing through the front face 2b and rear face 20, a first centrifugal compressor wheel 8 and a second centrifugal compressor wheel 10 mounted back-to-back at each end of the shaft 7, said first compressor wheel 8 constituting a first compression stage and said second compressor wheel 10 constituting a second compression stage. In particular, the shaft 7 is hollow in this embodiment and encloses a threaded rod 11, to each end whereof one of the compressor wheels 8, 10 is screwed, enabling the compressor wheels to be easily mounted and removed. Thus, the two compressor wheels 8 and 10 are driven on the same shaft 7, which improves the energy efficiency and avoids the need for a gearbox. The rear of the compressor wheels 8 and 10 includes a labyrinth seal to control the pressures in the compressor and balance the axial forces.

[0040] The casing 2 further encloses an electric motor that is preferably synchronous, positioned between the first compressor wheel 8 and the second compressor wheel 10 and arranged to rotate the shaft 7. The motor comprises a stator 14 and a rotor structure 16 which interact with one another to form a synchronous electric motor with at least one permanent magnet 16a (brushless motor). More particularly, the stator 14 is formed by a coil 14a and two ferrite elements 14b, mounted such that they are fixed relative to the casing 2. The rotor structure 16 comprises one or more permanent magnets 16a made integral with the shaft 7, for example by bonding, and is covered by a lining 16b. Flanges 16c are attached (for example by bonding) to the lateral ends of the lining and ensure that the magnet is resistant to the centrifugal forces at high speeds.

[0041] It should be noted that the workpiece intended to be rotated about a longitudinal axis A-A of a centrifugal compressor comprises at least one shaft 7, on which or in which a rotor structure 16 with at least one permanent magnet 16a of an electric motor is mounted or attached in order to drive the shaft such that it rotates about the longitudinal axis A-A, and an air or gas axial bearing 24 attached to one end of the shaft 7 or forming a part of the shaft 7. The workpiece can further comprise a first compressor wheel 8 and a second compressor wheel 10 mounted at two ends of the shaft 7, a front air or gas radial bearing 18 mounted on a first end of the shaft 7, and a rear air or gas radial bearing 22 mounted on a second end of the shaft 7, as shown in FIG. 2a.

[0042] As described below with reference to FIGS. 10 and 12, the machining method is carried out in a machining unit 100 which is adapted to receive the workpiece and in particular the shaft 7 with the axial bearing 24, and comprises a tool holder 110 with a tool for machining the ribs or grooves on at least a portion of the shaft 7 of the workpiece.

[0043] The shaft 7 is mounted such that it can rotate in the casing 2 about the longitudinal axis A-A thereof by means of at least one front radial bearing 18, one rear radial bearing 22 and one axial bearing 24. The centrifugal compressor 1 comprises a front radial bearing support 26 for carrying the front radial bearing 18, and a rear radial bearing support 28 for carrying the rear radial bearing 22, arranged to be positioned around the shaft 7, respectively at the front and rear of the motor. At the rear, a volute 29 is also provided between the rear radial bearing support 28 and the rear cover 3c. The volute 29 comprises the orifice leading to the tangential fluid outlet 6, after compression. An axial bearing support 30 is also provided to carry the axial bearing 24, arranged to be positioned around the shaft 7, between the first compressor wheel 8 and the front radial bearing support 26. It is clear that the axial bearing could be provided at the rear of the motor.

[0044] The bearings are non-contact, aerodynamic type bearings, in order to generate little friction. They require no lubrication and very little maintenance. More particularly, with reference to FIGS. 2a, 2b and 4, the axial bearing 24 is an aerodynamic bearing and is constituted by a disc including, on at least one of the faces thereof, first grooves 24a, preferably spiral-shaped over an annular area at the periphery, arranged to create a film of air. Preferably, the axial bearing 24 comprises preferably spiral-shaped grooves or ribs 24a at the periphery of the disc of the axial bearing 24 on a front face and on a rear face, obtained by a machining process explained below. The orientation of the grooves or ribs 24a can be different on the front face to that on the rear face or can be identical. The axial bearing 24 with its grooves or ribs 24a keeps the rotating shaft 7 longitudinally centred by generating films of air from the front face and rear face. The front radial bearing 18 and the rear radial bearing 22 are aerodynamic bearings, and the shaft 7 has, facing the front 18 and rear 22 radial bearings, second grooves or ribs 32 arranged to create a film of air or gas when the shaft is rotated in the air or gas radial bearings.

[0045] In FIGS. 2a, 2b and 4, the first centrifugal compressor wheel 8 and the second centrifugal compressor wheel 10 can still be seen mounted back-to-back at each end of the shaft 7. At the two ends of the shaft 7, there is a first portion of machined ribs or grooves 32 and at the other end a second portion of machined ribs or grooves 32. The rotor structure 16 with at least one permanent magnet 16a is fixed on the shaft in a central position for the electric motor.

[0046] It is also possible to have a rotor structure 16, which comprises one or more diametrically magnetised permanent magnets 16a bonded, shrunk inside the shaft 7 or disposed in a lining 16b. The one or more permanent magnets 16a can be shrunk into the lining 16b with as much rigidity and interference as possible to compress each magnet in order to compensate for the orthoradial pull resulting from centrifugal forces.

[0047] It should also be noted that the clearance between the shaft 7 and the one or more magnets 16a mounted on the outside of the shaft 7 must be sufficient to prevent any clamping of the magnet 16a against the shaft 7 at high speed and high temperature, which would put the inside of the magnet 16a in a state of high bi-axial tension and thus increase its risk of breaking. More specifically, the radial displacement of the inner surface of each magnet 16a can be less than the radial displacement of the outer surface of the shaft 7 due to the negative thermal expansion coefficient of the magnet in a radial direction. This is illustrated by the hatched area in FIG. 3, where the inner surface of the magnet 16a approaches the outer surface of the shaft 7 at the end of the rapid deceleration of the rotor structure 16 when it is still hot.

[0048] FIG. 3 gives a graph showing the difference between a radial displacement of the outer surface of the shaft 7 and that of the inner surface of the one or more permanent magnets 16a according to the invention. x corresponds to the direction of diametrical magnetisation of the magnet and y corresponds to the orthogonal direction in which the magnet contracts radially during heating. In the case of mounting by bonding, this difference corresponds to the radial deformation to be elastically accommodated by the adhesive between the shaft 7 and the permanent magnet 16a.

[0049] A bond can be formed between the shaft 7 and the magnet 16a, if the adhesive can withstand the radial deformation (up to 10 ?m seen in FIG. 3) under tensile stress at high temperature and at high speed. In this case, if the adhesive warps at operating temperature (for example at 150? C.) or if its modulus of elasticity decreases by several orders of magnitude, it no longer fulfils its load-bearing role and thus becomes useless.

[0050] In FIGS. 2a and 4, flanges 16c are attached to the lateral ends of the lining 16b in order to guarantee the mechanical integrity of the magnet 16a or of a plurality of magnets 16a in parallel side by side on the shaft 7 and that are subjected to centrifugal forces. The flanges 16c can be bonded or shrunk between the shaft 7 and the lining 16b.

[0051] Adhesive-free assembly is achieved by optimising the shrink fit (choice of materials and interference) of the flanges 16c on the shaft 7, the flanges 16c in the lining 16b and the lining 16b on the magnet 16a. Torque is thus transmitted from the magnet 16a or each magnet 16a to the shaft 7 via the flanges 16c. An adhesive-free assembly guarantees better rigidity than adhesive bonding, particularly at high speed and temperature where an assembly produced through adhesive bonding would see the frequency of its first bending mode decrease dangerously towards the rotation frequency.

[0052] The lining 16b or casing can ideally be made of carbon fibre and alternatively of a titanium or molybdenum alloy. The flanges 16c are ideally made of titanium alloy and alternatively of non-magnetic steel. The diametrical interference of the flanges 16c in the lining 16b is ideally the same as or slightly less than that of the lining 16b on the one or more magnets 16a.

[0053] It should also be noted that if the lining 16b and the flanges 16c are made of the same material, the lining 16b can be combined with at least one of the two flanges 16c to form a single part, particularly for mounting the permanent magnet structure 16a on an external side of the shaft 7. A flange 16c can also be replaced with a shoulder made on the shaft 7 (shrink fit banking for the flange, and axial positioning of the magnet).

[0054] As explained below, in particular in FIG. 10, although the grooves or ribs obtained by turning the shaft can be produced on the shaft in the bare state, it is nevertheless preferable to produce them after the assembly consisting of the lining 16b, the permanent magnet 16a and the flanges 16c has been mounted on the shaft 7 in order to guarantee the best geometric tolerances for the two aerodynamic bearings 18 and 22 shown in cross-section in FIG. 2a.

[0055] Although the grooves or ribs obtained by laser ablation on the shaft can be produced before or after the assembly consisting of the lining 16b, the permanent magnet 16a and the flanges 16c has been mounted on the shaft 7, it is preferable to produce these grooves or ribs after the axial disc 24 has been shrunk onto the shaft 7 if the magnet 16a is inside the shaft 7. In any case, it is essential that the final geometry of the bearings (and optionally of the assembly with the axial disc if assembled before finishing) is obtained before laser ablation.

[0056] Although the magnet 16a can be magnetised before it is mounted on the shaft 7, it is preferable to magnetise it after mounting, measuring and balancing the rotor structure 16 in order to facilitate these steps.

[0057] A first alternative embodiment is shown in FIG. 5 by way of a longitudinal sectional view, along the axis A-A, of the shaft 7 with the compressor wheels 8 and 10, at least the axial bearing 24 integral with the shaft 7 and the rotor structure 16 with one or more permanent magnets 16a according to the invention disposed inside the shaft 7, which is in tubular form. In this case, the one or more permanent magnets 16a are shrunk inside the shaft 7 or mounted with a small radial clearance. It should also be noted that the clearance between the rod 11 and the one or more magnets 16a mounted inside the shaft 7 must be sufficient to prevent the magnet 16a from being clamped against the rod 11 at high speed and high temperature, which would place the inside of the magnet 16a in a state of high bi-axial tension and thus increase its risk of breaking. The rod is threaded at its two ends 11b and 11d so that it can be screwed firstly into an internal thread 8b of the first compressor wheel 8, and secondly into an internal thread 10b of the second compressor wheel 10. The shoulder 11c is provided in the case of radial play between the shaft 7 and the magnet 16a in order to axially secure the magnet 16a and the threaded rod 11 after the compressor wheel 8 has been screwed on. By tightening the compressor wheel 10, the threaded rod 11 and the shaft 7 are axially secured to one another.

[0058] When the magnet 16a is inside the shaft 7 and the material of the shaft 7 is too fragile to allow shrink-fitting, the magnet 16a is secured to the shaft 7 either by bonding or by axial clamping via a rod placed under tensile stress. Axial clamping can also be produced on a solid magnet by machining threads directly in the shaft in order to screw in the wheels which will hold the magnet in the hollow shaft 7.

[0059] The hollow magnet is thus axially compressed slightly between the shoulder of the rod 11b and the tubular end 8b of the wheel 8. Tightening the wheel 8 compensates for the thermal expansion mismatch between the rod 11 and the magnet 16a. Tightening the wheel 10 compensates for the thermal expansion mismatch between the rod 11 and the shaft 7 and maintains sufficient adhesion to transmit torque.

[0060] FIG. 6 shows a second alternative embodiment of a longitudinal sectional view, along the axis A-A, of the shaft 7 with the compressor wheels 8 and 10, at least the axial bearing 24 integral with the shaft 7 and the rotor structure 16 with one or more permanent magnets 16a disposed inside the hollow shaft 7. This alternative embodiment combines bonding and axial clamping for assembling the almost solid magnet 16a inside the shaft 7. To do this, rods 11b and 11d, threaded at one end only and provided with shoulders at the other end, are bonded on their smooth part in the almost solid magnet 16a and screwed on the other side into the wheels. As a result, the magnet undergoes axial traction to ensure that the wheels are preloaded on the shaft 7 to allow torque to be transmitted by adhesion.

[0061] FIG. 7 shows an embodiment of the air or gas axial bearing 24. As can be seen in this FIG. 7 in plan view, ribs or grooves 24a of a certain depth are produced on an annular area starting from the periphery of the disc and leading towards the centre of the disc. The ribs or grooves 24a and the arrangement thereof are programmed, in particular in the machining unit, to activate the machining tool so as to produce all of the ribs or grooves at once, i.e. by moving the tool holder, or the rotating disc in a single machining direction, for example from the periphery of the disc to the bottom of the annular area of the ribs or grooves. Starting, for example, from the periphery of the disc on one face, groove portions are gradually formed by reciprocating motions of the machining tool in synchronism with the machining unit rotating the disc at a given speed according to a sinusoidal program or function.

[0062] With this program and in combination with the controlled reciprocating motions of the machining tool, each start of all of the grooves 24a is achieved by the rotation of the disc and the reciprocating motions of the machining tool. This is continuously repeated for the following groove or rib portions continuously from the first groove portion to the end or to the bottom of the annular area. With such a way of producing the different ribs or grooves 24a on the disc 24, the machining time per grooved face of the disc is less than one minute, which is significantly different from a previous machining technique using a laser beam.

[0063] FIG. 8 shows a first portion of ribs or grooves 32 produced on a first end of the shaft 7 with a first air or gas radial bearing 18 shown via a longitudinal section on the first portion of ribs or grooves 32 produced by the machining method.

[0064] As with the production of the grooves or ribs on the axial disc described hereinabove, during the machining of these grooves or ribs 32, the shaft 7 is rotated by the machining unit about the longitudinal axis thereof and moved along the longitudinal axis thereof in a machining direction. The machining tool disposed in its tool holder is moved back-and-forth opposite the first portion of the shaft to be machined at a frequency that depends on how the machining unit was programmed. It should be noted that instead of moving the shaft 7 in a longitudinal direction, the tool holder could be moved in the longitudinal machining direction. All of the ribs or grooves 32 are obtained at once by the machining tool on the first workpiece portion of the first end of the shaft driven in rotation, said machining tool carrying out reciprocating motions from the beginning to the end of the first workpiece portion of the shaft 7.

[0065] The machining unit is programmed to have a simultaneous synchronised rotation of the shaft 7 with the machining tool according to a sinusoidal program to obtain a determined arrangement of the ribs or grooves 32 on the first workpiece portion of the shaft 7 for the front air or gas radial bearing 18. Starting, for example, from the start of the first workpiece portion of the shaft 7 on the same side as the first end thereof and in one machining direction only, and as far as the end of the first portion of the first end of the shaft 7, all of the ribs or grooves 32 are machined at once, which significantly reduces the machining time.

[0066] It can therefore be determined that the sinusoidal function does indeed achieve synchronisation between the rotation of the shaft 7 and the displacement in a longitudinal direction of the machining tool or of the shaft 7. The frequency and amplitude of the sinusoidal function are chosen as a function of the geometry of the grooves 32, the number of grooves 32, the speed of rotation of the shaft and the speed of the longitudinal displacement.

[0067] The specific arrangement of the ribs or grooves 32 produced on the first portion of the first end of the shaft 7 is programmed in the machining unit. In one desired embodiment, the ribs or grooves 32 are each V-shaped, i.e. they include a change of orientation in principle from the middle of the first workpiece portion of the shaft 7. This ensures that the shaft rotating at high speed in the compressor is held without mechanical contact in the air or gas journal bearings. From as low as 6,000 rpm, the air or gas pressure in each aerodynamic radial bearing is such that the shaft is no longer in mechanical contact with the static journal bearing, thus avoiding any mechanical friction.

[0068] FIG. 9 shows the shaft 7 with a first portion containing machined ribs or grooves 32 on the first end of the shaft and a second portion on the second end of the shaft 7. The first portion and the second portion are machined in the same way as described above with reference to FIG. 6. The same arrangement of V-shaped ribs or grooves 32 at the two ends of the shaft 7 can be provided.

[0069] As a supplement to FIG. 1, the compressor 1 comprises a casing 2, made of aluminium, the top face 2a whereof is closed by a top cover 3a and the front face 2b and rear face 2c whereof are closed by a front cover 3b and a rear cover 3c respectively. The side faces 2d of the casing are joined at their base to form a bottom 2e with a U-shaped cross-section.

[0070] The top cover 3a is positioned on the same side as the electronic components of the compressor. Thus, access to the electronic components integrated in the compressor is easy, which access is provided through the top cover 3a. The front and rear covers 3b, 3c are used to reach the interior of the compressor (motor, rotor, bearings, etc.). A gasket is interposed between the top face of the casing 2 and the top cover 3a. This gasket protects the electronic components from dust and moisture.

[0071] The casing 2 has an inlet 5 for the fluid to be compressed provided on the front cover 3b and a tangential outlet 6 for the compressed fluid provided on one of the side faces of the casing 2.

[0072] In FIG. 1, the casing 2 comprises an inner housing extending coaxially to the longitudinal axis A-A from end to end between the front face 2b and the rear face 2c of the casing 2 and receiving the front radial bearing support 26 and the front radial bearing 18, the motor and the rotor structure 16 thereof attached to the shaft 7, the rear radial bearing support 28 and the rear radial bearing 22, the second compressor wheel 10 and the volute 29. On the front face 2b side, the inner housing is closed by the front cover 3b which integrates the first compressor wheel 8, the axial bearing support 30 and the axial bearing 24. On the rear face 2c side, the inner housing is closed by the rear cover 3c.

[0073] Moreover, at least one orifice is advantageously provided, for example the point given the reference 57a, arranged to allow the fluid to be compressed circulating within the channels to enter the motor and circulate between the stator 14 and the rotor structure 16, and at least one orifice, for example the points given the reference 57b, arranged to allow the fluid to be compressed to exit the motor and rejoin said channels after having cooled the motor.

[0074] Similarly, at least one orifice is advantageously provided, for example the points given the reference 59a in FIG. 1, arranged to allow the fluid to be compressed circulating within the channels 54 to circulate in the vicinity of the axial bearing 24, the front radial bearing 18 and the rear radial bearing 22, and at least one orifice corresponding, for example, to the same points given the reference 57b, arranged to allow the fluid to be compressed to rejoin said channels 54 after having cooled said axial bearing 24, front radial bearing 18 and rear radial bearing 22.

[0075] Thus, after entering the first compression stage through the inlet 5, the fluid to be compressed passes in the channels 54 through the parts of the compressor located along the longitudinal axis between the first compression stage and the second compression stage to rejoin the second compression stage. As a result, the fluid to be compressed, as it passes between the inner wall 52 and the ferrite elements 14b of the motor, cools the latter and recovers lost heat from the motor to increase the efficiency thereof before entering the second compression stage. Moreover, the orifices 57a, 57b, 59a allow for a slight deviation of the flow so that the fluid to be compressed also circulates between the stator 14 and the rotor structure 16 and in the bearings in order to cool these elements and recover the heat losses from the motor and the heat losses due to friction in the bearings.

[0076] The centrifugal compressor 1 allows very high rotational speeds to be reached, lying in the range 100,000 rpm to 500,000 rpm. It allows the fluid compressed in the first compression stage to pass substantially through the entire system to recover any waste heat, and in particular waste heat from the motor, bearings and electronic components, in order to increase the efficiency thereof before entering the second compression stage (as the temperature of the fluid to be compressed increases, so does the pressure thereof). Moreover, the use of the only fluid to be compressed to cool the compressor, without the aid of an additional cooling circuit, as well as the arrangement of the electronic components in the compressor for the electronics to be integrated into the casing, results in a very compact compressor. The compressor according to the invention thus has a high rotational speed and a high compression ratio while occupying a small volume. For example, a compressor according to the invention has a compression ratio greater than 3, and a power of the order of 4 kW for dimensions L?W?H in cm of the order of 14?8?11 for a weight of only 1.6 kg.

[0077] For example, the compressor according to the invention can be used with air or gas to power fuel cells, or any other system that uses compressed air (industrial compressors, medical compressors, ships, etc.).

[0078] With a refrigerant gas, the compressor according to the invention can be used in a mobile HVAC (heating, ventilation and air conditioning) system, such as in electric, hybrid or hydrogen vehicles.

[0079] The centrifugal compressor can also be used in a stationary system with a refrigerant gas such as a heat pump.

[0080] The centrifugal compressor can also be used with a natural gas.

[0081] FIG. 10 shows an overview of the machining unit 100 for producing ribs or grooves on portions of the first end and the second end of the shaft 7.

[0082] The machining unit 100 comprises a lathe 130 with a spindle 140 for holding the shaft 7 and rotating it when machining the ribs or grooves. According to an alternative embodiment, the spindle that holds the shaft in rotation can move in a longitudinal machining direction to produce the ribs or grooves as shown by the arrow Sm.

[0083] The machining unit 100 further comprises a tool holder 110 connected to the structure of the lathe 130. The tool holder 110 carries a machining tool 120, the machining head whereof, which comes into contact with the shaft to machine the grooves or ribs, can be moved back and forth according to how the machining unit is programmed. The head of the machining tool 120 can be made of diamond for machining the ribs or grooves on the shaft 7 made of tungsten carbide or ceramic. This is carried out so as to have a simultaneous synchronised rotation of the shaft 7 with the machining tool according to a sinusoidal program to obtain a determined arrangement of the ribs or grooves on the workpiece portions of the shaft 7 for each front or rear air or gas journal bearing.

[0084] In an alternative embodiment, the tool holder 110 can also be moved in a longitudinal machining direction instead of the shaft 7 for machining the ribs or grooves on the shaft.

[0085] The frequency of the reciprocating motions of the tool can also be changed according to how the machining unit 100 is programmed.

[0086] FIG. 11 shows a precision rib or groove for air or gas bearings obtained by the reciprocating motions of the machining tool, which is determined by the material to be machined, the speed of rotation of the shaft for the machining operation, the diameter of the shaft and many other parameters.

[0087] Finally, FIG. 12 shows a machining unit 100 for producing ribs or grooves on one or two faces of the disc of the air or gas axial bearing 24. This machining unit 100 comprises the same elements as those described with reference to FIG. 10 and will not be repeated with reference to this FIG. 12. Only the manner in which the tool holder 110 is connected to the structure of the lathe 130 should be noted.

[0088] It goes without saying that the present invention is not limited to the examples shown and that various alternatives and modifications that may be clearly apparent to a person skilled in the art can be made thereto. Other combinations are of course possible with what is already known with centrifugal compressors. Fast and precise machining of ribs or grooves can be carried out on other workpieces than those described hereinabove with equivalent machining elements.