Patent Application
20240369068
This application is claiming priority based on European Patent Application No. 23171496.5 filed on May 4, 2023.
The invention relates to a method for machining ribs or grooves for air or gas bearings of a rotary-shaft-comprising workpiece of a high-speed centrifugal fluid compressor, as well as to a method for assembling the components of the workpiece. The compressor is a two-stage compressor, and comprises a casing with a fluid inlet and a compressed fluid outlet and encloses a shaft rotatably mounted about a longitudinal axis. A first compressor wheel and a second compressor wheel are mounted back-to-back on the shaft, the first compressor wheel constituting a first compression stage and the second compressor wheel constituting a second compression stage. The centrifugal compressor further comprises a motor, preferably a synchronous electric motor, positioned between the first compressor wheel and the second compressor wheel and arranged to rotate the shaft. At least one air or gas axial bearing is mounted at one end of the shaft, and a front radial air or gas bearing is mounted on a first end of the shaft and a rear radial air or gas bearing is mounted on a second end of the shaft.
The invention further relates to the workpiece obtained by the method for machining and/or assembling the components of the workpiece.
Fluid compressors are typically referred to as turbocompressors or centrifugal compressors. They comprise a stator and a rotor forming a permanent magnet synchronous motor (brushless motor). They can reach very high speeds, such as 100,000 to 500,000 rpm. The motor drives the compressor wheels at high speed, at which the compressor wheels compress the fluid. The fluid can be air, a gas, a refrigerant or any other suitable fluid. By using two compressor wheels, the fluid is compressed twice as much.
These compressors can be used, for example, in a mobile HVAC (heating, ventilation and air conditioning) system with a refrigerant gas, such as in electric, hybrid or hydrogen-powered vehicles. These compressors can also be used in a stationary system with a refrigerant gas, such as a heat pump.
These compressors typically comprise a first circuit for circulating the fluid to be compressed and a second circuit for circulating a cooling liquid used to cool the compressor, and more particularly the motor and the air or gas bearings supporting the motor shaft on the one hand, and the electronic components on the other. More specifically, rotating the motor at high speed causes it to heat up considerably, so much so that the compressor components need to be cooled to prevent damage thereto. These circuits are typically provided inside the compressor itself, at least as far as the cooling circuit is concerned. There is no provision to facilitate the flow of the cooling gas or air during the operation of the compressor, in particular at high speeds, which constitutes a drawback. Moreover, the air or gas bearings supporting the rotor shaft are not designed to support the rotor shaft without friction, which leads to significant heat generation when the rotor is rotating at a high speed, which constitutes another drawback.
Moreover, grooves or ribs are known to be made on the air or gas bearings for air or gas flow, and to create pressure and cooling. However, the grooves are produced without any specific arrangement by laser machining, which constitutes a drawback because the machining time is too long and thus the costs thereof are too high.
One purpose of the invention is to overcome the various drawbacks mentioned hereinabove by means of a method for rapidly producing grooves or ribs on a rotor shaft of a workpiece at each air or gas radial bearing, and on an air or gas axial bearing attached to the shaft or forming part of the shaft. Said grooves in the shaft are arranged in such a way as to overcome gravity when the shaft of the compressor rotates at high speed in each bearing and to allow the rotating rotor shaft to be held without mechanical contact on an air or gas current in the radial bearings and thus virtually frictionlessly.
To this end, the present invention relates to a method for machining ribs or grooves for air or gas bearings of a rotary-shaft-comprising workpiece of a high-speed fluid compressor, which comprises the features of independent claim 1.
Specific steps of the method are defined in dependent claims 2 to 8.
To this end, the present invention further relates to a method for machining ribs or grooves on a workpiece comprising a rotary shaft, as well as to a method for assembling the components of the workpiece according to independent claims 9 and 10.
To this end, the present invention further relates to a workpiece intended to be rotated about a longitudinal axis of a centrifugal compressor and intended to have ribs or grooves machined according to the machining method, which workpiece is defined in independent claim 11.
Specific embodiments of the workpiece are defined in dependent claims 12 to 15.
One advantage of the method for machining ribs or grooves on a rotary-shaft-comprising workpiece of the compressor in a machining unit is that all the ribs or grooves are obtained at once on a workpiece portion driven in rotation, by a machining tool which moves back and forth from the beginning to the end of the workpiece portion of the rotary-shaft-comprising workpiece. To do this, 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 arrangement of the ribs or grooves to be produced on the workpiece portion.
The sinusoidal function therefore makes it possible to achieve synchronisation between the rotation of the spindle or shaft and the movement in a longitudinal direction of the machining tool or spindle. The frequency and amplitude of the sinusoidal function are chosen as a function of the geometry of the grooves, the number of grooves, the speed of rotation of the shaft and the speed of the longitudinal displacement.
During the machining of the rotary-shaft-comprising workpiece, the workpiece or the tool holder carrying the machining tool is also displaced in a longitudinal machining direction while the machining tool carries out the reciprocating motions.
The reciprocating motions of the tool are compared to a piezoelectric oscillator where the oscillation frequency can be varied to speed up or slow down the reciprocating motions of the tool according to how the machining unit was programmed to obtain said desired ribs or grooves. During the reciprocating motions, the tool is in the machining position and in contact with the workpiece at one time and not in contact with the workpiece at another time.
The workpiece mainly comprises a shaft attached to a rotor structure of an electric motor driving it in rotation, or at least one air or gas axial bearing mounted at a first end of the shaft or forming a part of the shaft, between a compressor wheel and an air or gas radial bearing. Ribs or grooves can be produced on one face or preferably on both faces of the disc of the air or gas axial bearing. Alternatively, they can be produced on one face of the static axial bearings facing the axial disc.
One advantage of the method for machining ribs or grooves in a machining unit is that the ribs or grooves are produced very quickly in less than 1 minute on each workpiece portion of the shaft for the air or gas radial bearings and with great precision. The same applies to the grooves or ribs produced on one or both faces of the disc of the air or gas axial bearing driven rotatably. The machining tool is harder than the material of the shaft or of the air or gas axial bearing.
As explained above, the machining unit can be programmed for simultaneous synchronised rotation with the machining tool according to a sinusoidal program to obtain a rib or groove arrangement on each workpiece portion of the shaft for each air or gas radial bearing, and for the air or gas axial bearing.
As a result of such a machining of the ribs or grooves, which are preferably V-shaped with a change of orientation of each rib or groove in the centre of each machined portion on the shaft, the shaft rotating at high speed can be held in the compressor in the air or gas radial bearings without mechanical contact. The shaft is thus held in each radial bearing with virtually no friction by the pressure of the air or gas passing within the grooves or ribs as a result of the high-speed rotation of the shaft. 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 radial bearing, thus avoiding any mechanical friction. It goes without saying that the faster the shaft rotates, the greater the air pressure in the radial bearing, which automatically generates more air or gas friction.
The grooves or ribs are machined by the machining unit and the machining tool according to a sinusoidal program or function and the desired arrangement of the grooves or ribs, so as to reverse the orientation of the grooves or ribs on the shaft that is substantially towards the inner half of each static radial bearing placed above said ribs. This generates an air or gas pressure, which can become greater and greater the faster the shaft rotates.
Such a high-speed centrifugal fluid compressor can rotate at very high speeds without excessive heating due to the production of the ribs or grooves on the air or gas radial bearings.
A axial bearing is also provided between the first compressor wheel and the first radial bearing. Grooves or ribs are made at the periphery in the form of spirals on a front face and a rear face of the disc of the axial bearing. Films of air are generated by the grooves as the shaft rotates to hold the shaft in a longitudinally well-centred position.
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:
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.
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.
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
As described below with reference to
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.
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
In
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.
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
A bond can be formed between the shaft 7 and the magnet 16a, if the adhesive can withstand the radial deformation (up to 10 μm seen in
In
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.
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.
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).
As explained below, in particular in
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.
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.
A first alternative embodiment is shown in
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.
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.
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.
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.
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.
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.
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.
As a supplement to
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.
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.
In
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.
Similarly, at least one orifice is advantageously provided, for example the points given the reference 59a in
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.
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.
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.).
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.
The centrifugal compressor can also be used in a stationary system with a refrigerant gas such as a heat pump.
The centrifugal compressor can also be used with a natural gas.
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.
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.
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.
The frequency of the reciprocating motions of the tool can also be changed according to how the machining unit 100 is programmed.
Finally,
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 23171496.5 | May 2023 | EP | regional |
