US20150192035A1

US20150192035A1 - Device with rotor, stationary part or stator, and different types of liquid pocket sliders with respective specific functions

Info

Publication number: US20150192035A1
Application number: US14/417,182
Authority: US
Inventors: Pasquale Dell'aversana
Original assignee: ASTRO IND Srl
Current assignee: ASTRO IND Srl
Priority date: 2012-07-24
Filing date: 2013-07-18
Publication date: 2015-07-09
Also published as: EA201590253A1; EP2877752A1; CN104541076A; WO2014016739A1; ITRM20120355A1

Abstract

A device includes a rotor (5), a stator (2) and different types of liquid pocket sliders (3; 4; 10), each type with different and specific functions, the liquid pocket sliders preferably being mounted at the outermost surface of the rotor. The device can include ordinary pocket sliders (3), with the function of load support and bearing self-centering on the rotor (5); “sentinel” pocket sliders (4) having the function of predicting failures and/or monitoring the distribution of the load around the rotor; preload pocket sliders (10), with the function of compensating for possible unbalance of the rotor at a given velocity.

Description

FIELD OF THE ART

The present invention generally regards the technical field relative to the devices comprising a rotor, a stationary part arranged around the rotor, and bearings that serve for supporting and allowing the rotation of the rotor with respect to the stationary part.
Specifically, the present invention relates to a device of this type, in which the particular arrangement of the bearings, and their nature and conformation, allow solving a series of problems that cannot be remedied with the common ball bearings or with rollers or other bearings of the state of the art such as foil bearings. The solved problems will all be discussed in the following description.
In general, by “stationary part” or “stator”, it is intended a part that is fixed with respect to a machine (e.g. a turbomachine)—said stator directly surrounding the peripheral part of the rotor—while the term “rotor” indicates a rotating part integral with the rotating shaft, and whose peripheral surface directly faces the stationary part. The rotating shaft constitutes the innermost rotating part of the device; of course, it is coaxial with the periphery/circumference of the rotor.
For example, if the device of the invention constitutes a device mounted in a turbomachine (turbine, compressor, pump, fan, etc.), the stationary part corresponds with the casing, while the rotor of the device is formed by the actual turbine and by all the rotating members integral therewith; for example, in the case of a CMG (Control Moment Gyro) satellite device for controlling the attitude of a satellite, the rotor comprises the flywheel while the stationary part comprises the stator of the motor (usually electric) and the box for containing the flywheel.

PRIOR ART AND PROBLEMS THEREOF

For any one rotating machine essentially constituted by a rotor, a stator and bearings, it is important to prevent excessive vibrations and oscillations of the axis of the rotor system. This requirement can be particularly important in the case of turbomachines (turbines, compressors, pumps, fans, etc.), in which the interspace or clearance between the stator and the rotor is usually very small, in order to reduce to a minimum the return of the flow and thus optimize the compression ratio. A functioning that lacks vibrations is very much desired in any case in order to reduce the noise, improve the efficiency of the machine and lengthen the operative life thereof. More generally, the higher the rotation velocity and the moment of inertia of the rotor, the more important it is to have a good balancing of the rotor and a good rotor-stator alignment. In fact, it is understood that the effect of an unbalance, even if small, of the rotating set could be amplified in an intolerable manner at high velocities, generating dangerous vibrations with the increase of amplitude, accelerating the wear both on the bearings and on the shaft, and possibly leading to resonances for specific critical velocities which in the end could cause the collapse of the entire system. One example of a device in which this problem is particularly relevant is constituted by the mechanical gyroscopes, in which relatively large flywheels rotate at considerably velocities. With regard to this device category, an extreme situation is that relative to the inertial wheels, to the stabilizer wheels, and to the CMG (Control Moment Gyros) devices, used for controlling the attitude of a satellite. In these case, in fact, the rotating element (the flywheel) must work in an environment with very low pressure, sometimes for many years of continuous functioning, without the direct intervention of man, and at high velocities (typically between 3000 and 10000 rpm). Consequently, the bearings and the relative lubrication system constitute crucial elements in these spatial apparatuses. The lubrication system of the CMG spatial devices, for example, must be able to feed the correct quantity (usually very little) of lubricant in order to reliably ensure a very long functioning lifetime, to compensate for the degradation and wear of the lubricant, and to prevent the wear both of the bearings and of the shaft, or even the final seizure. In addition, since the bearings belonging to the prior art, used in the spatial devices for controlling the attitude, are usually roller bearings or ball bearings, their rotating elements must be mechanically processed with extreme precision and hardened by special anti-wear coatings. They must not introduce any vibration that could be harmful or extremely dangerous for the correct functioning of the satellite. In addition, such bearings must be capable of resisting the mechanical stresses that occur during launch, without sustaining any damage.
In order to solve at least some of these problems, the apparatuses of the prior art, not only the spatial ones, often provide for the use of very hard and precise bearings, a structure characterized by rather heavy shafts as well as fairly complex lubrication systems. Indeed, the shafts and the bearings of the conventional apparatuses have the function of exchanging the torque with the rotor as well as the function of maintaining the amplitude of the vibrations of the rotor at the lowest possible level. In practice, this involves the use of very heavy machines, with a high inertia of the shaft, with a rather poor ratio between the power developed and the mass, and consequently with a high consumption of fuel and lubricant, without mentioning the high manufacturing costs. In addition, if on one hand the shafts are designed and manufactured as heavy, bulky parts, on the other hand the ball bearings are higher performing if arranged around small shafts; this signifies that the smaller the radius of the shaft, the greater its maximum functioning angular velocity (in rpm). Indeed, the greater the linear velocity of the bearing, the greater the heat generated locally with consequent loss of viscosity of the lubricant and loss of its load capacity. Hence, with the ball bearings, high angular velocities of the shaft are only possible using small radii, in order to maintain the liner velocity at a suitable value. Therefore, the requirements of the bearings may be antithetical to the requirements of the machine's overall performances, and most of the time acceptable compromises must be reached.
Finally, the automatic functioning (without direct human intervention) of the CMG device for controlling the attitude of the satellite implies that automatic means must be attained for detecting the start of performance deterioration, possibly due to the wear of the bearings system. These means are often rather complex and are capable of detecting the start of the performance deterioration when the system has already been damaged to a certain extent. Indeed, they detect the consequences in a mechanism that has already been damaged, such as the vibrations (noise) or increased friction (by means of an increased power required to maintain the rotation velocity at a constant value). In those cases in which the vibrations are monitored for detecting the incipient degradation of the performances, piezoelectric accelerometers are often used as detectors. Such sensors/detectors are often fixed on the bearings or on the stator, but the problem then becomes that of being able to distinguish between the frequencies to be associated with different conditions. Sometimes these means are not implemented and the failure of the CMG device occurs suddenly, with dramatic consequences on the functioning of the satellite; other times, even if these automatic means are employed for detecting the incipient malfunctioning, it could be too late to enact suitable countermeasures.
Hence, object of the present invention is to provide a new type of structure of a generic rotating machine, and particularly of a device with flywheel for controlling the attitude of a satellite, which allows solving the above-illustrated problems of the prior art.

DESCRIPTION OF THE INVENTION

In the device according to the present invention, a particular type of bearing is used in an original manner, non-obvious even for a man skilled in the art; such bearing, already known in the general shape thereof, is formed by pocket sliders called liquid pocket bumpers described in the patent application WO 2004/053346 A1 by the same author. The prior art also comprises other examples of bearings or pocket sliders based on a similar functioning principle, but it no case known to the author have devices similar to LPS sliders been used in the modes and for the functions described in the present invention. For example, the patent U.S. Pat. No. 4,170,389, by A. Eshel, regards (see FIG. 17) a thrust bearing constituted by pockets filled with a fluid or particles of various morphology, built for tolerating, without damage, the passage of an impurity that has inserted itself between the shaft and the block of the bearing. Or, (see FIG. 4) the liquid pocket bearing 87 or 89 has a toroidal shape and acts once again as an axial thrust bearing for a discoid plate 83 integral with the shaft 82. Such structures are also intended for tolerating small misalignments of the shaft, nevertheless they are not adapted to carrying out the function of stabilizing the shaft, nor for carrying out any diagnostic function or failure prevention function. Even the bearings described in WO 2004/053346 and used in the present invention, like those described in U.S. Pat. No. 5,114,244, by J. L. Dunham et Al., only serve as thrust bearings, as direct support of the rotation shaft or as support elements for the sliding of a piston in a cylinder, while nothing is said regarding a preferred mounting position, which in the present invention is indicated as the outermost possible position, around the rotor, compatibly with other design constrains. In addition, neither WO 2004/053346, nor U.S. Pat. No. 5,114,244 specify the manner of designing these sliders with self-adaptive surface for maximizing the self-centering effects thereof on the rotor. Even the elimination of the vibrations and the stability of the system are obtained, in the present invention, in a non-random manner, only at specific conditions calculatable from the design in the manner which will be clarified below. In particular, the elimination of the vibrations due to an imperfect balancing of the rotor is obtained, in the present invention, through the introduction of a preload in phase with the centrifugal force deriving from the unbalance, due to elements specially set for carrying out this function. Finally, neither WO 2004/053346, nor U.S. Pat. No. 5,114,244 provide for the use of pocket sliders whose main function is not that of supporting a load but that of detecting an incipient failure in advance, i.e. that of functioning as sentinel elements in the manner described hereinbelow. The prior art also comprises other hydrodynamic bearings with self-adaptive surface. For example, there are the so-called foil bearings, whose structure is indeed different from that of the pocket sliders described in WO 2004/053346. Said foil bearings have several characteristics in common with said pocket sliders (e.g. the capacity to function at high velocities without liquid lubricants, etc.) but, at will be clear hereinbelow, they could never be used to carry out all the functions carried out by the devices used in the present invention (for example, they could not carry out the function of sentinel element, etc.) without considering that the foil bearings are of much more difficult and costly construction than the liquid pocket sliders or bumpers and that, unlike the latter, they must be custom-designed for each specific application.
In U.S. Pat. No. 3,456,993, by G. Müller, a bearing constituted by a membrane containing a pressurized liquid supports a flexible foil, whose face opposite that rested on the bearing is the active surface that supports a shaft in rotation. In such structure, the liquid pocket carries out a function analogous to that of the so-called bump foil of the foil bearings, but the system described herein is not designed for tolerating the breakage of the foil, nor for exerting a stabilizing and self-centering action, or for predicting incipient failures. In the following description, the particular bearings of the prior art that are employed in an original manner by the invention, in a plurality of different and specific functions, will be called liquid pocket sliders (Liquid Pocket Sliders, abbreviated with LPS) since such name appears technically more correct than the name Liquid Pocket Bumper (LPB) used in WO 2004/053346 for the same pocket sliders. The reasons will be underlined for which by employing these LPS according to a particular shape and arrangement, it is possible to obtain the following positive effects:

- tolerance and compensation of small misalignments between the rotor and the stator;
- elimination of the resonances;
- diagnostic control of the device functioning, for the purpose of preventing malfunctions;
- elimination of the vibrations due to an imperfect balancing of the rotor.

These problems are particularly important in the case of devices for controlling the attitude of a satellite. In such devices, the flywheel must be able to rotate for years without direct human surveillance, so that it is necessary to avoid the following as much as possible:

- a) an excessive unbalance, indicated below with “e”,
- b) a wear of the active surfaces (i.e. of the surfaces subjected to relative sliding, under load) caused by the vibrations and/or by the friction,
- c) unexpected functioning irregularities.

Specifically, the essential part of the invention consist of placing these special liquid pocket sliders—with a rest shape, a preload, and a rigidity that are specially designed (and possibly with special constraints for the membrane of the liquid pocket)—around the outermost periphery of the rotor, rather than placing conventional bearings (ball bearings, roller bearings, foil bearings, etc.) around the shaft of the rotor, and conferring particular characteristics to one or more liquid pocket sliders that allow them to work as “sentinel elements”, whose possible breakage does not involve any deterioration (or only a negligible deterioration) of the performances of the entire device (e.g. of a CMG, Control Moment Gyro satellite device).
In the following description, the theory underlying the present invention will also be described, together with a method for determining the preload and the characteristics of the liquid pocket LPS sliders most suitable for eliminating the vibrations and the resonances from the design.
A structure of the device like that according to the present invention would not be possible by employing conventional bearings, for the reasons indicated above. Indeed, rolling bearings would not function correctly at the extremely high linear velocities normally present on the periphery of the rotors rotating at high angular velocity; above all, even only one damaged rotating/rolling element—such as a ball or roller—would inevitably cause damage to the remaining part of the CMG device.
Hereinbelow, the main base characteristics of the LPS bearings of the prior art are listed:

- they are constituted by one or more sliders, rather than by rolling elements such as balls or rollers;
- such sliders are essentially formed by a flexible membrane with high elastic modulus, which is fixed on a rigid structure and closes, i.e. retains, a liquid;
- they can operate without liquid lubricants (when the slider slides on a relative slide surface it can create, due to viscous drag, a gap traversed by air or gas with a specific velocity profile, between the relative slide surface and the LPS slider, and such air or gas acts as lubricant);
- the lubrication occurs in a hydrodynamic manner due to the air/gas in the environment where the device operates, only above a critical velocity, which is a function of the properties of the gas, of the load conditions, of the materials, etc.;
- the active surface of these LPS sliders/bearings is the external surface of the membrane that encloses the liquid. It is very yieldable and hence, under the action of the lubricant, spontaneously assumes the most efficient shape possible (i.e. that which maximizes the load support capacity of the slider itself), which in turn depends on the distribution of the pressure generated inside said lubrication channel for any one functioning condition;
- at stationary functioning conditions and velocities, these sliders work according to a perfect hydrodynamic condition, i.e. in the absence of contacts with the active surfaces;
- the rigidity of the LPS sliders can be actively adjusted both when the device is at rest and when it works normally;
- their capacity of adaptation of the surface to the distribution of the pressures inside the lubrication channel is essentially independent of their rigidity. This implies for example that the rigidity of the LPS bearings can be modified without considerable variations in the load support capacity by the device at any given velocity;
- an essential difference with respect to the ball bearings is that their load support capacity increases with the increase of the functioning velocity, also because the heat produced is relatively little and hence the variations of the viscosity of the lubricant are relatively small;
- they are resistant to mechanical shocks to the extent in which the tensile strength of the membrane and the compressibility of the enclosed liquid are sufficient for absorbing such impact.

In the original structure proposed, any deflection of the axis of the rotor from its nominal direction is directly opposed by the LPS sliders/bearings, which tend to be more loaded at the points towards which the rotor is moved, so as to exert a self-centering action on the rotor. For small unbalancing of the rotor, the deflection of the rotor axis—and hence the load overload of several elements of the LPS bearing—can never increase excessively since the reaction force that each LPS is capable of rapidly exerting increases with the compression of such LPS element. In addition, the LPS element that has sustained one such small overload continues to work in hydrodynamic condition and is capable of reacting to the small deflections of the rotor without undergoing or causing an excessive increase of the friction and the heat generated on the affected surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of a mere non-limiting example by making reference to several particular embodiments thereof and to the relative theory/methodology, all shown in the closed figures, in which:

FIG. 1 shows a movement vs. frequency (or pulsation) graph, for a mass M fixed on a body with elastic constant k and subjected to an external force F(t) variable with frequency ω₁, the system assumed to have only one frequency thereof ω₀=(k/M)^1/2and only one degree of freedom, the movement being normalized with respect to the movement d₀that there would be if the force F was constantly applied with it maximum amplitude;

FIG. 2 shows a graph of the dependence of the ratio between the misalignment “d” and the unbalance “e” for a rotating system, on the ratio between the rotation frequency (angular velocity) ω₁of the rotor and the critical frequency (of resonance) ω₀of the system; also in this case, the treatment is simplified by considering only one degree of freedom of the system;

FIG. 3 shows a diagram illustrating the quality comparison, in the absence of preload on the rotor, between the effect due to the centripetal force (exerted by the LPS sliders on the rotor) and to the centrifugal force (it too acting on the rotor by virtue of the unbalance “e” and/or the movement=misalignment “d”), respectively; it is noted that for d=d* the sum of the forces equals zero;

FIG. 4 very schematically illustrates the liquid pocket slider (8), or LPS slider, at rest and under load, respectively (with a squeezing δ against a relative slide surface 2);

FIG. 5 shows various configurations, arrangements and constraints for the LPS sliders; specifically:

FIG. 5 a) shows, in front perspective, nine (=3×3) LPS elements with hemisphere shape, with constraints (6) also called containment or cage walls, considering, i.e. imagining, a specific square surface with area l²filled with said hemispheres; the constraints/walls (6) arranged around each hemisphere prevent any outward movement of the membrane (8 or 9) of the slider which retains the liquid in the relative hemisphere;

FIG. 5 b) shows (always imagining the relative plan view on an area l²) four spherical caps LPS lacking constraints;

FIG. 5 c) shows (always imagining the relative plan view on an area l²) a single spherical cap LPS lacking constraints and having greater size;

FIG. 6 shows a qualitative graph of the load (W) vs. squeezing (δ); it is noted that the smaller the radius of curvature, the greater the reaction to the squeezing; the letters a), b) and c) refer to the respective cases of the preceding FIG. 5;

FIG. 7 shows the total force f_Ton the rotor (given by the sum of the centripetal reaction of the LPS bearing, shown above the abscissa, and the centrifugal force, below the abscissa, due to the unbalance “e” and to the movement or misalignment “d” of the rotor with respect to the position of the symmetry axis of the rotor in the rest state) for a system of sliders, i.e. of LPS elements sufficiently preloaded for working at a maximum angular velocity ω_Mand in the presence of an unbalance “e” of the rotor itself; it is known that the greater the design velocity ω_M, the greater the required preload will be; in addition, so that f_Thas a minimum in d=0, the greater the unbalance “e”, the greater the rigidity “dW/dδ” must be of the system LPS after preloading;

FIG. 8 is a schematic section view (orthogonal to the axis of the shaft of the rotor) of a particular structural shape of the device of the present invention, in a possible embodiment, with 4 LPS liquid pocket sliders of “sentinel cell” type (4), arranged in “strategic” positions (here at intervals of 90°) in order to obtain information regarding the distribution of the load during the rotation of the rotor (5);

FIG. 9 is a view of an enlarged detail of FIG. 8, which in particular shows the constraints (or containment or cage walls) (6) that must be imagined to completely surround each element LPS (3; 4; 10), and a sensor (7) associated with a LPS pocket or sentinel cell (4).

FIG. 10 is a view of a detail that represents the preloading cells (10) installed on the rotor (5) and provided with a generic device for adjusting the preload (11) and with a sensor (7) that can for example be a pressure sensor or a load cell.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

Explanation of the Self-Centering Functioning Principle of the Flywheel (or Rotor)

Generally, in mechanical or electromechanical systems, the causes of the vibrations are of various type. Consequently, each of these systems is usually characterized by multiple frequencies thereof.
Nevertheless, for the sake of simplicity, in the examples the case of only one degree of freedom will be examined. In any case, it is deemed that this is sufficient for illustrating the underlying principle of the invention.
d₀is the movement of a mass M fixed to a body with elastic constant k, when a constant force F₀is applied thereto. If a force is applied to this system, but with an amplitude F(t) that varies with a frequency ω₁between 0 and F₀, then the movement d of the mass M will be given by the formula:
$\begin{matrix} d = \frac{d_{0}}{1 - {(\frac{ω_{1}}{ω_{0}})}^{2}} where ω_{0} = \sqrt{\frac{k}{M}} . & (1) \end{matrix}$
When ω₀=ω₁the system is in resonance (see graph of FIG. 1).
Now, a rotor is considered in which:
M=mass of the rotor
{right arrow over (e)}=distance of the center of mass of the rotor from the axis of the rotor (unbalance)
{right arrow over (d)}=ideal movement/misalignment of the axis of the rotor from the rotation axis
{right arrow over (ω₁)}=angular velocity of the rotor
{right arrow over (ω₀)}=critical frequency of the system.
In this case, the rotor will be subjected to a centrifugal force which, in addition to rotating with an angular velocity ω₁, has an intensity dependent on the modulus of this angular velocity. Indeed: {right arrow over (F_c)}=Mω₁ ²({right arrow over (e)}+{right arrow over (d)}).
{right arrow over (ω₁)} and {right arrow over (ω₀)}, are generally different from each other since {right arrow over (ω₀)} depends on the characteristics of the system while {right arrow over (ω₁)} is set by an external actuator. Nevertheless, the entire system will oscillate with the same frequency as the centrifugal force, i.e. ω₁.
For a given unbalance e, F_cgrows linearly with the movement d of the axis of the rotor and as such constitutes a destabilizing force.
Any stabilizing force capable of opposing such centrifugal force in the machines of the prior art must be applied to the rotor by means of the bearings and the shaft, which therefore require being as rigid and balanced as possible. The rotors supported by magnetic supports are an exception, which nevertheless imply an active control and much more complex embodiments. The present invention instead allows designing rather light rotor systems, with simple structure and without requiring active control, at the same time relaxing certain requirements of the bearings.
If the system is not in resonance, an elastic force F_e=kd may be capable of balancing the centrifugal force.
Hence, in equilibrium conditions:
$F_{c} = M ω_{1}^{2} (d + e) = kd = F_{e} . \frac{d}{e} = \frac{M ω_{1}^{2}}{k - M ω_{1}^{2}}$ $with ω_{0} = \sqrt{\frac{k}{M}} one obtains \frac{d}{e} = \frac{\frac{ω_{1}^{2}}{ω_{0}^{2}}}{1 - \frac{ω_{1}^{2}}{ω_{0}^{2}}}$
Hence, for ω₁>>ω₀the rotor will rotate around its center of mass, and for ω₁=ω₀the system will be in resonance. (see FIG. 2).
Hence, even in the presence of a small unbalance, a linear elastic reaction to the centrifugal force gives rise to vibrations and resonances. A shaft of the rotor which for small inflections obeys Hooke's law, for example, constitutes an elastic system that has such behavior. Analogously, bearings of foil bearings type, even mounted on the external periphery of the rotor, would not resolve the problem of the resonances (given that their reaction to the load is approximately linearly proportional to d, in the simplest structures).
In the present invention, the shaft does not have the task of generating an elastic reaction to the centrifugal force acting on the rotor; rather, most of this function is completed by the liquid pocket sliders (LPS) mounted on the periphery of the rotor and which will be described hereinbelow.
Now, the elastic reaction of such LPS sliders or bearings is not linear. For small deformations, it can be approximated by means of the formula: F_LPS≅k_LPSd².
Hence in equilibrium conditions we have:
$M ω_{1}^{2} (d + e) = k_{LPS} d^{2}$ $k_{LPS} d^{2} - M ω_{1}^{2} d - M ω_{1}^{2} e = 0$ $\frac{d}{e} = \frac{M ω_{1}^{2} \mp \sqrt{M^{2} ω_{1}^{4} + 4 k_{LPS} M ω_{1}^{2} e}}{2 k_{LPS} e}$
Now, setting
$ω_{*} = \sqrt{\frac{2 k_{LPS} e}{M}},$
one has:
$\begin{matrix} \frac{d}{e} = \frac{1}{2} \frac{M ω_{1}^{2}}{k_{LPS} e} \mp \sqrt{\frac{M^{2} ω_{1}^{4}}{4 k_{LPS}^{} e^{2}} + \frac{2 M ω_{1}^{2}}{2 k_{LPS} e}} = \frac{ω_{1}^{2}}{ω_{*}^{2}} \mp \sqrt{\frac{ω_{1}^{4}}{ω_{*}^{4}} + \frac{2 ω_{1}^{2}}{ω_{*}^{2}}} Hence :  \frac{d}{e} = \frac{ω_{1}^{2}}{ω_{*}^{2}} (1 \mp \sqrt{\frac{\frac{ω_{1}^{2}}{ω_{*}^{2}} + 2}{\frac{ω_{1}^{2}}{ω_{*}^{2}}}}) & (2) \end{matrix}$
Therefore, when ω₁>>ω_*, the following solutions are obtained:
$\begin{matrix} \frac{d}{e} = - 1 & a) \\ and \\ \frac{d}{e} = 2 \frac{ω_{1}^{2}}{ω_{*}^{2}} & b) \end{matrix}$
in which the formula a) is obtained by applying l'Hôpital's rule.
Solution b) does not have a physical significance since the system is externally constrained. Consequently, d=−e i.e. the rotor tends to rotate once more around its center of mass, such that, with e≠0, it vibrates.
The only difference with respect to the preceding case (centrifugal force balanced by an elastic linear force of F_e=kd type) is that resonances are no longer in the picture.
This is already a good result, but the objective is that of eliminating the vibrations as well. The solution consists of a suitable preloading of the LPS sliders or bearings.
It must be observed that such preloading must always be in phase with the centrifugal force, which rotates with frequency ω₁. Hence, in the embodiments of the present invention in which it is desired to eliminate even the vibrations of limited amplitude, in addition to the resonances, the simplest thing to do will be to obtain several special cells, with the function of preloading cells (10), directly on the rotor, with the relative slide surface obtained on the internal surface of the stator. Of course, if the other LPS elements (3; 4) are fixed to the internal surface of the stator (2), the preloading cells (10) must be fixed on the rotor (5) in a different axial or radial position in order to not interfere with the other LPS elements (3; 4) during rotation. Each of such preloading cells will be provided with a pressure sensor like the sentinel cells 4, as well as with preloading means, as in claim 7, adapted to adjust the internal pressure thereof in a manner independent from all the other cells. In this manner, the preloading cells, fixed in a sufficient number on the external surface of the rotor, will rotate integrally therewith, necessarily in phase with the centrifugal force to be balanced.
Operatively, the preload necessary for eliminating the vibrations at a given design speed ω_Mcan be obtained by employing the following procedure:

- 1. all the preloading cells are pressurized at a same initial pressure;
- 2. the rotor is made to rotate at the velocity ω_Mand the pressure variation produced inside each preloading cell is measured, such variation due to the non-uniform distribution of load due to unbalance;
- 3. the rotation is stopped, and through the aforesaid preloading means, a pressure distribution similar to that detected when the rotor rotated at the velocity ω_Mis obtained inside the preloading cells.

At this point, the rotor will be balanced at the velocity (D_M.
Let's examine, mathematically, how a suitable preload is capable of eliminating all the vibrations.
Set k_LPSd²=Mω_M ²e, and seek the value d* that compensates for the centrifugal force (=Mω_M ²e) for a null movement.
This corresponds with the desired preloading.
One obtains d*=±√(Mω_M ²e/k)=±ω_M√(Me/k) with ω_M=maximum design velocity, and k≡k_LPSfor simplifying the formulas.
The preloading condition for the LPS bearings becomes:
Mω ₁ ²(d+e)=k(d+ω _M√(Me/k))² (3)
By solving the quadratic equation (3) in d, one obtains a discriminant
Δ² =M ²ω₁ ⁴−4 √kω _M√(Me)Mω ₁ ²+4kMeω ₁ ², from which
for ω₁→ω_MΔ²=(M·ω_M ²−2√kω_M√(Me))²and a solution, always for ω₁→ω_M
d=(2kω _M√(Me/k)−Mω _M ²±Δ)/2k i.e. d ₊=0.
From this it follows that for ω₁→ω_M, d→0.

Instead,

$d_{-} = 2 ω_{M} \sqrt (Me / k) - \underset{k}{{\underline{M ω}}_{M}^{2}}$
The latter solution is to be discarded since the rotor system is constrained and d therefore cannot assume large values.
In addition, however, the rotor must also be stable.
Hence, for d=0 it must hold true that the total force
F(d)=k (d+ω_M√(Me/k))²−Mω₁ ²(d+e) meets the minimum condition, i.e.:
F′(d=0)=2 ω_M k√(Me/k)−Mω ₁ ²=0
i.e.:
from which, by writing
4ω² _M kMe=M ₂ω⁴
$\frac{ω_{M}^{} e}{d^{* 2}}$
in place of k:
$\frac{4 ω_{M}^{4} M^{2} e^{2}}{d^{* 2}} = M^{2} ω_{1}^{4}$

- and therefore:

$\begin{matrix} d^{*} = 2 {e (\frac{ω_{M}}{ω_{1}})}^{2} & (4) \end{matrix}$
Therefore, in order to prevent all vibrations, the preload d* must be greater the greater the unbalance and the further away the work frequency from the maximum design frequency. We recall that the condition (4) is valid in our simplified approximation that the resultant of the forces that are opposed to the centrifugal force is of the type F_LPS≅k_LPSd². Nevertheless, it is clear that the idea underlying the invention remains valid even if the precise form of said resultant of the forces that is opposed to the centrifugal force is slightly different.
The same concepts can be graphically illustrated since this constitutes a qualitative though perhaps more direct method.
In addition, the following considerations can be useful for understanding the role carried out by the shape of the LPS sliders and the reason for which the correct selection of this shape is so important.
With reference to FIG. 3 enclosed with the present patent application, for the sake of simplicity it is assumed that ω₁=ω*=ω (less favorable condition) and that, at rest, the rotor is perfectly aligned with respect to the central line thereof, i.e. that d(ω=0)=0. It is assumed that the LPS bearings barely touch the rotor, without any preloading against the latter. Then, when the system begins to rotate, d tends to increase until it reaches the value given by the preceding equation (2) (1+√3)e, and the centrifugal force will do the same. With the increase of d, the centripetal reaction of the LPS bearings also increases, but with a different law (˜d²), and finally it balances or compensates for the centrifugal force.
The distance d* (FIG. 3), for which the centripetal reaction of the LPS sliders or bearings balances or compensates for the centrifugal force on the rotor, can be canceled out by suitably preloading these LPS sliders, as was shown above. In such a manner, for a given velocity of the rotor, the minimum energy U of the system is reached when the axis of the rotor coincides with the central line of the system, also in the presence of a small unbalance or misalignment of the same rotor (see FIG. 7, total force f_T).
The system is stable at the minimum of the curve:
U(x)=−∫_x ₀ ^x F(ξ)dξ+U(x ₀)
where F(ξ) is the total force as a function of the movement.
Since such reasoning is valid, the tilt or slope of the centripetal reaction curve of the LPS liquid pocket thrust bearing must be greater than the slope of the centrifugal force due to the unbalance/misalignment, at the point where the two absolute values compensate each other.
Such condition is verified so long as one correctly selects the shape of the LPS bearings. With the term “suitable preloading” of the LPS sliders, it is intended that one must take under consideration all of these factors.
Now, while for e≠0, the absolute value of the centrifugal force for d=0 varies like ω², the absolute value of the centripetal reaction of the LPS sliders is only determined by the preloading and does not vary with the rotation velocity. Hence, it always remains extremely important to have well-balanced rotors available.
That said, even a relatively large misalignment of the axis of the rotor (due to a relatively flexible shaft in the presence of transverse gravity, for example) can be tolerated since it can be more easily compensated at low velocities (i.e. immediately after starting, before the machine reaches greater velocities). Certainly this constitutes an important advantage with respect to the conventional machines, since the requirement relative to the rigidity of the shaft can be less severe.
A suitable preload can only be established by taking under consideration the shape of the LPS liquid pocket sliders or bearings and their internal pressure.
The pressure jump across the membrane is given by the generalized Laplace formula,
$Δ P = τ (\frac{1}{R_{1}} + \frac{1}{R_{2}})$
where R₁and R₂are radii of curvature of the surface of the membrane and the tension of the membrane depends on the surface area of the membrane (this does not hold true for liquids, where the surface tension does not depend on the surface area of the liquid).
The load that the air film (or air channel formed between the LPS bearing and the relative slide surface) must sustain is approximately given by the product between the area of the flattened part of the membrane (contact area) and the pressure drop across the membrane. The preload is determined by the tension τ of the membrane, by the initial shape of the non-deformed membrane (without the load) and by the squeezing δ of the LPS slider against the relative slide surface (for example the surface of the flywheel); see FIG. 4.
The tension also depends on the elastic modulus E, which constitutes a property of the material. Hence, for any one given material, and a defined initial shape, the graph of the load (W) can be designed as a function of the squeezing (δ).
Hereinbelow in the present description, reference will be made to FIG. 5 and to the corresponding FIG. 6.
The following example shows that a particular geometric configuration affects the rigidity of the LPS system, for a given area l².
The present invention, indeed, regards a self-centering rotor in which the geometric configuration of the LPS systems mounted on the periphery (edge) of said rotor is selected in a manner so as to optimize the rigidity of the LPS slider or bearing, in order to obtain a stable system at the maximum possible value of ω and for a maximum of the estimated value of the unbalance “e” (such unbalance “e” must in any case be reduced to the minimum possible during the manufacturing of the rotor).
In other words, such geometric configuration tends to be that which maximizes the variation of the curvature of the surface of the membrane 8 of the LPS bearing, for a specific squeezing (δ), with respect to the system not subjected to the load. The present invention also includes those cases in which the condition of optimization (maximization) of the curvature of the surface for a given squeezing (δ) with respect to the non-loaded system, is also obtained by taking under consideration possible design requirements/parameters, which could lead to a shape of the LPS membrane different from the ideal theoretical form.
With reference to FIG. 5, a specific square surface with area l²can be filled with different LPS configurations, as the abovementioned figure shows. The LPS pocket sliders of case a) are constrained by a wall (6) that prevents any movement of the membrane (8) (of the LPS slider) towards the outside, so as to oblige the membrane (8) itself to bend, with a lower radius of curvature than an analogous non-constrained membrane (8). See for example the diagrams of FIG. 6 with regard to the behavior under load of such systems of LPS sliders. In FIG. 6, it is also noted that the smaller the radius of curvature, the greater the reaction to the load W. The letters a), b) and c) refer to the cases of FIG. 5.
Now, returning to FIG. 5, as stated above, the containment walls 6 of case a) oblige the membrane (8) to be bent more than a membrane that is not constrained. In order to correctly carry out its function, said containment walls must be tangent to the surface of the membrane (8) along its fixing line, i.e. along the peripheral line where the membrane (8) “comes out” from the rigid structure to which it is fixed. Normally, such rigid structure is movable so to be able to move the slider from the rest condition (FIG. 4, left side) towards the preloading condition against the relative slide surface (FIG. 4, right side), or the preloading could be obtained by increasing the internal pressure of the liquid (“swelling” or injecting liquid) inside the pocket of the slider.
Finally, FIG. 7 shows the total force f_Tacting on the rotor, given by a system of liquid pocket sliders (LPS) or bearings having a preload sufficient for being able to work at a maximum design angular velocity ω_Mand in the presence of an unbalance “e”; it is observed that the total force f_T(vectorial sum of the centrifugal force and the centripetal reaction) must have a minimum for d=0 in order to ensure the stability of the rotor 5.
In FIGS. 8 and 9, an embodiment is shown in which the liquid pocket sliders (LPS sliders) (3) are mounted on the stator (2), while the external (cylindrical) surface of the rotor (5) constitutes the relative slide surface, i.e. the surface which faces directly on the internal (cylindrical) surface for mounting the LPS bearings on the stator (2). First of all, it is observed that also the numbers (4) and (10) indicate liquid pocket sliders (LPS sliders or bearings), but that these (even if they substantially have many characteristics in common with the LPS sliders indicated with the reference number (3) in this specific embodiment of the invention) constitute the so-called “sentinel cells” (4) or the so-called preloading cells (10), whose function will be clarified hereinbelow in a thorough manner.
In addition, it is observed that the configuration for mounting the LPS sliders (3) and (4) could be reversed with respect to the embodiment of the invention shown in FIGS. 8 and 9; i.e. said sliders (3) and (4) could be arranged on the external surface of the rotor (5), which would then constitute the mounting surface that faces the internal surface of the stator (2), the latter then representing the relative slide surface.
Before describing the functioning and significance to be associated with the sentinel cells (4), it is observed that in order to have a specific initial preloading of the LPS sliders against the relative slide surface, it is suitable to provide for mechanical means that allow adjusting such preload. The preloading means of the liquid pocket sliders (3; 4) advantageously comprise a movable structure (for the sake of simplicity not shown in FIG. 8 and FIG. 9), on which the LPS slider is directly mounted, as well as mechanical actuators (e.g. a small piston, also not represented in FIGS. 8 and 9) in order to press the relative LPS slider (3; 4; 10), causing the squeezing (6) thereof against the relative slide surface (or according to the modes stated above, and as specified in claim 7).
The sentinel cells (4) and their essential advantages will now be described in more detail.

Tolerance for Possible Failure and Opportune Detection of an Incipient Deterioration.

A certain number of liquid pocket sliders, pressurized independently from each other, arranged around a rotor in the above-described manner, are capable of obtaining a failure-tolerant system of LPS sliders. Damage caused to the single balls of a conventional ball bearing, due to a failure of the check means or to the wear or degradation of the lubricant, could cause a rapid deterioration of the performances and in the end even a series of functioning irregularities of the entire mechanism. Damage or even breakage of a single, independently-pressurized liquid pocket of a LPS bearing, would however lead to the immediate depressurization of the membrane (8) or (9) that is broken, which therefore would no longer sustain the load, but would no longer produce damage to the rotor (5).
In this manner, if a sufficient number of pressurized pockets remain pressurized, the system would not undergo any damage and the mechanism can continue to function.
If a sufficient number of LPS sliders all work together simultaneously, the breakage of only one of these involves only a negligible increase of the load for the remaining sliders, so that the performances of the system remain substantially the same. The tolerability of the breakage of a single liquid pocket element was already shown in WO 2004/053346. In the present invention, the concept is used by introducing LPS sliders with the single, specific function of preventing failures; such function was not contemplated in the previously cited invention.
Indeed, several LPS sliders can be specially obtained in a manner so as to be weaker than the other LPS sliders, which we will call “ordinary LPS sliders”; the weaker LPS sliders will be called “sentinel cells”.
Such sentinel cells can be obtained in a manner so as to have limited size, in order to support a total load percentage that is as low as possible, reinforcing the concept according to which the failure of a single sentinel cell does not affect the overall performance.
Under certain aspects, a sentinel cell functions as a fuse of an electrical apparatus, in the sense that the breakage of the sentinel cell indicates that an operating limit has been exceeded, given that the sentinel cell is specially made to be the weakest component of the system.
An important difference with respect to the electrical fuses nevertheless consists of the fact that if a LPS sentinel cell “burns”, this does not stop the system, since the system can be failure-tolerant in the sense clarified above. On the contrary, the breakage of a sentinel cell simply signals the need to substitute/renew the system of bearings.
Another difference with respect to the electrical fuses is that the LPS sentinel cells do not serve to protect the remaining part of the system from damage: they represent diagnostic means rather than protection means.
The sentinel cells 4 work in parallel with the other ordinary sliders (3), while an electrical fuse works in series. The LPS sentinel cells (4) can also sustain a breakage due to a process of progressive degradation due to the prolonged functioning, rather than following an unsupportable/unsustainable variation of their operative conditions.
In the case of automatic functioning without direct human supervision, such as for the bearings of a CMG device for controlling the attitude of a satellite, the failure of a sentinel cell could thus make a possible system failure predictable, thus placing the control from Earth in the conditions for adopting appropriate countermeasures.
An opportune signaling of the incipient deterioration of a LPS bearing provided with sentinel cells is easily obtainable by monitoring the LPS sentinel cells, so as to readily detect their possible irregularities. Such monitoring can be implemented by means of sensors (7), as indicated in FIG. 9. For example:

- a pressure sensor, capable of detecting the variation of the pressure inside the LPS membrane, would immediately signal the breakage of the membrane and its consequent depressurization;
- a load cell could be used for detecting the variation of the load due to the breakage of the membrane;
- optical means could detect the significant variations of the shape of the membranes of the sentinel cells, which could be the consequence of a breakage thereof;
- a temperature sensor, which measures the temperature of the liquid inside the LPS sentinel cell, could indicate an irregular variation of the temperature as a consequence of the loss of the liquid.

It is underlined that the above-listed means for detecting the breakage of a sentinel cell or of its irregular functioning are to be considered as merely exemplifying. Thus, it is easily comprehensible that the use of other detection means in the sentinel cells could not be claimed as an invention in itself.
In principle, the radial load on the LPS elements does not have to be distributed in a uniform manner all around the flywheel. Therefore, several LPS elements could be more stressed than others. In order to account for such possibility, the sentinel cells will be positioned in key, i.e. strategic positions, in order to monitor the operating conditions at several points of the system.
Indeed, by providing such sentinel cells with specific sensors, the system would have the further characteristic of being capable of detecting irregularities or non-nominal functioning conditions. For example, a pressure sensor in each sentinel cell could monitor the relative internal pressure, which in turn is correlated with the local conditions of the load. This would constitute an element/important piece of information, since a non-uniform load distribution around the flywheel could cause a premature breakage of several liquid pocket sliders (LPS sliders) of ordinary type (i.e. in addition to the breakage of a sentinel cell). Thus, the function of the sentinel cells would not be limited to supplying information only of “yes/no” type: they could also supply a more complete view of the general functioning conditions of the device, making possible the integration of their information with other data/information elements coming from other sources.
Therefore, the present invention has various aspects and advantages that distinguish it from the prior art. While a flywheel of the prior art was supported by ball bearings or roller bearings of conventional type around the rotation shaft thereof, according to the present invention the same flywheel is automatically supported and centered by a plurality of independent LPS bearings, in the sense that the breakage of one of these causes at most the outflow of a small quantity of liquid which is dispersed in the surrounding environment without interfering with the functioning of the other LPS liquid pockets. The heat generated in the sliders/bearings, in addition to being lower, is transferred and quickly dispersed through the liquid contained in the pockets. Once again, the breakage of a sentinel cell (which also carries out the function of bearing) signals the possibility of a failure to the system in a specific subsequent period, and hence the sentinel cells act as diagnostic means (in order to then take possible countermeasures) which do not interfere with the functionality of the system. The monitoring of suitable sensors associated with the sentinel cells, also in the absence of the breakage thereof, can supply useful information on the overall functioning, indicating for example irregular distributions of load between the various LPS elements. Finally, several cells provided with independent sensors and preloading means can be obtained on the rotor, rather than on the stator, so as to be able to introduce a preload always in phase with the centrifugal force connected with possible unbalancing of the rotor itself, in a manner so as to completely cancel the vibrations at a certain design velocity. Therefore, the present invention, applied in particular to a flywheel of a CMG device of a satellite, supplies many advantages not easily intuitable by the man skilled in the art.

Claims

1. Rotation device comprising:

a rotor (5) integral with a rotation shaft (1), a stator (2) coaxial with the rotor (5) and which faces directly on the external periphery of the rotor (5), forming an annular interspace with respect to the latter, liquid pocket sliders (3; 4; 10) comprising a membrane (8) for containing a liquid and functioning as radial bearings during the rotation of the rotor (5), said membrane (8) being characterized by a tension (τ) and by a high elastic modulus (E), i.e. substantially rigid upon traction, said rotation device providing for means for preloading the liquid pocket sliders (3; 4; 10) against a respective relative slide surface,

characterized in that said liquid pocket sliders (3; 4; 10) are mounted inside said peripheral annular interspace by fixing the edge of their membrane (8 or 9) to a mounting surface inside the stator (2) or to a mounting surface outside the rotor (5), the surface opposite the mounting surface constituting said relative slide surface; and wherein, given an estimated maximum of the possible unbalance (e) of the rotor (5), and given a maximum design angular velocity (ω_M) of the rotor (5), the centripetal reaction force exerted by said liquid pocket sliders (3; 4; 10) on the rotor (5) is such to prevent resonances by limiting the amplitude of the oscillations due to possible small misalignments between the rotor (5) and the stator (2) and/or due to the centrifugal force Mω²e for each rotation velocity ω≦ω_M, given that “M” is the mass of the rotor (5) including the shaft (1); and wherein the elastic rigidity (dW/dδ) of said liquid pocket sliders (3; 4) is such that the total force (f_T) acting on the rotor has a local minimum at d=0.

2. Device according to claim 1, characterized in that at least one of said liquid pocket sliders (3; 4) constitutes a sentinel cell (4) adapted to detect an excessive load or an excessive wear which has possibly caused the breakage of its membrane (9) over time.

3. Device according to claim 2, characterized in that said at least one sentinel cell (4) is associated with a pressure sensor (7) for continuously or intermittently measuring the internal pressure of the liquid retained by the membrane (9), so as to verify possible liquid losses outside the relative liquid pocket slider (4).

4. Device according to claim 2, characterized in that at least one of said sentinel cells (4) has smaller size than that of the other liquid pocket sliders (3) of ordinary type, which do not have the function of predicting possible device failures, and therefore such sentinel cell (4) supports a smaller percentage of the total load with respect to a liquid pocket slider (3) of ordinary type.

5. Device according to claim 2, characterized in that said sentinel cells (4) are arranged in strategic positions for constantly or intermittently monitoring the possible non-uniform distribution of the load on the rotor, i.e. around the rotor itself, given that they are preferably arranged at regular intervals from each other and between the liquid pocket sliders (3) of ordinary type.

6. Device according to claim 2, characterized in that each of said sentinel cells (4) is associated with one or more sensors (7) of different type, adapted to detect the breakage thereof and/or the variation of the sustained load, said sensors (7) being selected for example from among the following types:

pressure sensors;

load cells;

optical sensors;

temperature sensors.

7. Device according to claim 1, characterized in that said means (11) for preloading the liquid pocket sliders (3; 4; 10) comprise a movable structure and actuators for causing the relative compression between the slider (3; 4; 10) and said relative slide surface, or said preloading means (11) comprise means for varying the internal pressure of the liquid in the liquid pocket slider (3; 4; 10).

8. Device according to claim 1, characterized in that the rigidity (dW/dδ) of said liquid pocket sliders (3; 4; 10) is adjusted by employing one or more of the following measures:

adjusting the internal pressure of the liquid, by acting for example with a small piston that pushes the liquid from a tank communicating with the liquid pocket interior through a respective duct;

providing for a surface with curvature that is even non-uniform, but is adapted in the non-stressed rest condition for the surface of the membrane (8) of the liquid pocket slider (3; 4; 10);

providing for a suitable squeezing (δ) once the following are set: the materials of the liquid and the membrane (8) containing it, the internal pressure of the liquid, as well as the shape of the membrane (8) of the slider (3; 4; 10) for δ=0;

providing for cage/containment walls/constraints (6) along the edges of the membrane (8) at its edge for fixing to the mounting surface.

9. Device according to claim 8, wherein said cage/containment walls/constraints (6) constitute stops that limit the maximum oscillation amplitude of the rotor (5) and which are formed by materials which minimize the damage to said relative slide surface in case of contact.

10. Device according to claim 2, characterized in that a sufficient number of LPS elements (10) are fixed on the rotor (5) so as to be able to introduce a preload in phase with the centrifugal force due to the unbalance “e” of the rotor (5), capable of balancing said centrifugal force at a specific velocity.

11. Device according to claim 2, characterized in that the membrane (9) of the sentinel cells (4) is adapted to be broken before the membrane (8) of the other types of liquid pocket sliders (3; 10) by virtue of the fact that it has one or more of the following characteristics:

its thickness is calibrated in a manner so as to be smaller, e.g. 50% smaller, than that of the membrane (8) of the other types of liquid pocket sliders (3; 10);

its material is different and less resistant than that of the membrane (8) of the other types of liquid pocket sliders (3; 10);

it has an anti-wear and/or anti-friction surface coating with calibrated thickness smaller than that of the other types of liquid pocket sliders (3; 10);

it has an anti-wear and/or anti-friction surface coating of less resistant material than that of the other types of liquid pocket sliders (3; 10).

12. Device according to claim 1, characterized in that axial thrust bearings are also provided, or simultaneously both radial and axial thrust bearings are provided (radial/axial with regard to the thrust direction), wherein at least one of these, but preferably all, constitute liquid pocket sliders.

13. Device according to claim 1, characterized in that the cage/containment walls/constraints (6) provided along the edge of the membranes (8; 9) are tangent to the surface of the membranes (8; 9) at their fixing edges or lines.

14. Device according to claim 1, characterized in that it constitutes a so-called CMG (Control Moment Gyro) device for controlling the attitude of a satellite, wherein the rotor (5) comprises a flywheel (5) which is driven by a motor.

15. Device according to claim 14, characterized in that said liquid pocket sliders having the function of radial and/or axial thrust bearings during the rotation of the flywheel (5) are mounted on the external radial surface of the flywheel (5), which thus constitutes said mounting surface.

16. Device according to claim 1, characterized in that if the device constitutes a part of a turbo machine, a pump, a turbine, a compressor or the like, then the rotor (5) comprises, in addition to a system of blades or the like, also a ring whose external surface forms a mounting or slide surface for the liquid pocket sliders (3; 4; 10).

17. Device according to claim 1, characterized in that at least several liquid pocket sliders (3; 4; 10), but preferably all, are pressurized independently from each other, in order to make the device failure-tolerant.

18. Method for optimizing the functioning of a flywheel of a control moment gyros device and for opportunely preventing the failures thereof which comprises using the device according to claim 1.