Abstract:
The invention concerns a contact roller bearing for a vacuum pump comprising a rotor roller bearing ring ( 12 ) a coaxial stator roller bearing ring ( 13 ) between which are provided rolling elements ( 14   a,    14   b,    14   c,    14   d ) housed in succession and urged to roll on respective raceways of the rotor ( 12 ) and stator ( 13 ) roller bearing rings. The rolling elements comprise an alternating succession of rolling elements ( 14   a,    14   c ) whereof the outer surface is made of steel and of rolling elements ( 14   b,    14   d ) whereof the outer surface is made of ceramic, thereby reducing the resistance to acceleration of the contact roller bearing, which limits the friction and wear of the inner annular surface ( 16 ) of the contact roller and the corresponding bearing of the rotor.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    The present invention relates to rotor suspensions in vacuum pumps.  
           [0002]    In vacuum pumps, a rotor mounted to rotate in a stator is held by magnetic bearings which, in normal operation, keep the rotor in a radially centered position within the stator, holding it centered with specified accuracy and without mechanical contact between the rotor and the stator. Magnetic bearings comprise electromagnets that are powered electrically by appropriate circuits that servo-control the radial position of the rotor within the stator.  
           [0003]    The effectiveness with which the rotor is held radially within the stator is determined by the force of the electromagnets, and such holding requires the electromagnets to be powered with sufficient electricity.  
           [0004]    Sometimes the normal operation of magnetic bearings can be defective or insufficient, e.g. in the event of the rotor being subjected to a sudden large stress or in the event of the electrical power supply to the electromagnets being interrupted. Under such circumstances, the magnetic bearings no longer serve to center the rotor, and a “landing” step occurs in which the rotor passes from the state in which it is held without mechanical contact to a state in which it is held with mechanical contact. During such landing, the rotor tends to come into contact with the stator. Because the rotor is spinning very fast, e.g. about 30,000 revolutions per minute (rpm), such contact can cause the vacuum pump to be destroyed.  
           [0005]    To resolve that problem, proposals have already been made to fit vacuum pumps with secondary mechanical bearings for landing purposes that are based on ball bearings, which serve, in the event of the normal operation of the magnetic bearings failing, to limit radial displacements of the rotor within the stator and to provide approximate centering of the rotor while limiting radial movements of the rotor to a value that is smaller than the size of the airgap of the magnetic bearings.  
           [0006]    Nevertheless, the number of landings that are possible without significantly degrading the mechanical bearings remains limited, thus reducing the reliability of the vacuum pump and increasing the frequency with which maintenance operations are required.  
           [0007]    There exists a need to increase the number of landings that are possible and to increase the operational lifetime of mechanical landing ball bearings.  
           [0008]    The present invention results from the observation that certain failures of mechanical landing ball bearings are the result of the landing ball bearings presenting resistance to acceleration. In normal operation of magnetic bearings having mechanical landing bearings mounted on the stator, the balls in the mechanical landing bearings are stationary relative to the stator; when the magnetic bearings cease to operate, the rotor comes into contact with mechanical landing bearing inner rings that are still stationary, and imparts rotation both to the bearing inner rings and to the rolling parts situated between the inner rings and outer rings; because of the resistance to acceleration presented by the landing bearings, the rotary speed of the inner rings can increase only progressively, such that slip occurs between the rotor and the inner rings of the mechanical landing bearings. This inevitably results in the respective contacting surfaces of the rotor and the inner rings of the mechanical landing bearings being subjected to wear which progressively increases the clearance and reduces the effectiveness of the device; in addition, frictions between the various pieces can sometimes lead to shavings or filings appearing, which then run the risk of jamming the rolling parts of the mechanical bearing.  
           [0009]    Wear phenomena and their consequences are particularly accentuated when the inner rings of the mechanical landing bearings are subjected to phenomena that oppose rapid acceleration in rotation which would otherwise enable them to reach the speed of rotation of the rotor quickly.  
           [0010]    In this respect, a first cause of a landing bearing resisting acceleration is in its own inertia. Attempts have been made to reduce the inertia of landing bearings by using rolling elements of smaller mass. Thus, attempts have been made to replace traditional rolling elements such as balls of stainless steel with balls made of a ceramic of density that is well below that of steel. Under such circumstances, all of the rolling elements are ceramic balls. This leads to manufacturing cost being very much greater since ceramic balls are much more expensive than steel balls. A small improvement in landing bearing lifetime is indeed observed due to a reduction in the wear phenomena that occur between the rotor and the rings of the rotor bearing. This improvement nevertheless remains insufficient, particularly since it leads to significant extra cost. And this solution using ceramic balls presents another drawback of significantly reducing thermal conductivity between the stator and the rotor, and consequently of reducing the cooling capacity of the rotor.  
           [0011]    A second cause of resistance to acceleration in a landing bearing appears to be the friction that occurs between the adjacent rolling elements themselves, which friction appears to be accentuated by the very high accelerations to which the landing bearing is subjected during a landing. The invention takes advantage of this analysis and proposes a solution that enables such frictions to be reduced very significantly.  
         SUMMARY OF THE INVENTION  
         [0012]    Thus, the problem proposed by the present invention is that of designing a new structure for a mechanical ball bearing for landing purposes which presents lifetime that is increased so as to enable a larger number of landings and a longer duration of operation without failure.  
           [0013]    To do this, the invention provides a special structure that enables resistance to acceleration of the landing bearing to be reduced as much as possible, and in particular enabling the friction forces which occur between the moving elements of the landing bearing to be reduced, which forces are liable to resist acceleration of the landing bearing during landing.  
           [0014]    Another object of the invention is to reduce the cost of producing landing bearings for vacuum pumps, by limiting the use of expensive materials.  
           [0015]    In the invention, it is essential for these effects to be obtained without having recourse to lubrication means using liquids that could pollute the vacuum created by the vacuum pump.  
           [0016]    To achieve these objects, and others, the invention provides a landing bearing structure for a vacuum pump comprising:  
           [0017]    a rotor ring and a coaxial stator ring which define a roller housing between them;  
           [0018]    rolling elements housed one after another in the roller housing and rolling on respective running tracks of the rotor and stator rings;  
           [0019]    the rolling elements comprising an alternating succession of rolling elements having outside surfaces made of steel and of rolling elements having outside surfaces made of ceramic.  
           [0020]    In a simplified embodiment, the rolling elements are spherical balls.  
           [0021]    Preferably, the steel rolling elements are made of stainless steel, while the ceramic rolling elements are made of silicon nitride.  
           [0022]    The running tracks can be made of stainless steel.  
           [0023]    The rolling elements can all have the same diameter when the ball bearing is operating under normal temperature conditions. Normal operating temperatures are usually about 60° C. to 90° C. For this purpose, provision is made for the ceramic rolling elements to be slightly larger in diameter, at ambient temperature, than the steel rolling elements so as to compensate for differences between the coefficients of thermal expansion of ceramic and of steel.  
           [0024]    Alternatively, the ceramic rolling elements can be slightly smaller in diameter than the steel rolling elements under normal operating temperature conditions. In practice, it is possible for example to use rolling elements which all have the same diameter at ambient temperature.  
           [0025]    A vacuum pump of the invention has at least one landing bearing as defined above.  
           [0026]    For example, such a vacuum pump comprises a rotor mounted to rotate in a stator, with at least one radial magnetic bearing which, in normal operation, holds the rotor in a radially centered position inside the stator, and with at least one mechanical landing bearing with a landing ball bearing which, in the event of normal operation of the radial magnetic bearings failing, limits radial displacement of the rotor within the stator by ensuring that the rotor remains approximately centered, radial clearance being provided between one of the rotor or stator rings and the corresponding bearing surface of the rotor or of the stator. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    Other objects, characteristics and advantages of the present invention appear from the following description of particular embodiments given with reference to the accompanying figures, in which:  
         [0028]    [0028]FIG. 1 is a general longitudinal section view of a vacuum pump whose rotor is held by magnetic bearings and by associated mechanical bearings for landing purposes;  
         [0029]    [0029]FIG. 2 is a detail section view on a larger scale of zone A in FIG. 1, showing half of a mechanical landing bearing in an embodiment of the present invention;  
         [0030]    [0030]FIG. 3 is a face view on a larger scale of a landing bearing constituting an embodiment of the invention; and  
         [0031]    [0031]FIG. 4 is a perspective view in partial section of the FIG. 3 landing bearing. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]    In the embodiment of FIG. 1, a vacuum pump comprises in general manner a stator  1  having an axial suction inlet  2  and a radial delivery outlet  3 . A rotor  4  is mounted to rotate axially inside the stator  1  about the longitudinal axis I-I. The rotor  4  has a suction system represented by fins  5 , and a shaft  6  turning in bearings of the stator  1 . In the figure, there can be seen two radial magnetic bearings  7  and  8 , and two mechanical bearings for landing purposes  9  and  10  comprising radially-acting ball bearings. For the record, there can also be seen an axial magnetic bearing  11 .  
         [0033]    In normal operation, i.e. in the absence of excessive stress on the shaft  6  of the pump and assuming that the magnetic bearings are themselves operating normally, the magnetic bearings hold the rotor  4  correctly in an axially-centered position, and the mechanical landing bearings  9  and  10  do not touch the shaft  6 .  
         [0034]    In the mechanical landing bearing  9 , there can be seen a rotor bearing ring  12 , placed close to and around the shaft  6  of the rotor  4 , and a coaxial stator bearing ring  13  placed in contact with the stator  1 . Between them, the rotor ring  12  and stator ring  13  define a running housing  19 . Rolling elements  14  such as balls, needles, or any other conventional type of rolling element, are received in the running housing  19  between the rotor ring  12  and the coaxial stator ring  13 , so as to constitute a rolling bearing allowing the two rings  12  and  13  to rotate axially relative to each other.  
         [0035]    Reference is now made to FIG. 2 which shows half of the mechanical landing bearing  9  in greater detail and on a larger scale in situation between the shaft  6  of the rotor  4  and a corresponding portion of the stator  1 . There can be seen a rolling element  14  in the running housing  19  between the rotor ring  12  and the coaxial stator ring  13 . The rolling element  14  rolls on respective running tracks  20  and  21  of the rotor and stator rings  12  and  13 . There can also be seen the radial magnetic bearing  7  which, in normal operation, serves to center the shaft  6  of the rotor  4  in the stator  1  leaving an empty annular airgap  15  that defines the maximum radial displacement available for the shaft  6  inside the stator  1 . Under normal circumstances, the airgap  15  can be about 0.2 to 0.4 mm wide, for example. The purpose of the mechanical landing bearing  9  is to limit the maximum radial displacement available to the shaft  6  of the rotor  4  inside the stator  1  to a value which is considerably smaller than the size of this airgap  15  so as to avoid damaging the magnetic bearings in the event of a landing.  
         [0036]    Between the inner annular face  16  of the rotor ring  12  and a corresponding first bearing surface  17  of the rotor  4 , radial clearance  18  is left that is considerably smaller than the airgap  15  and that is only slightly greater than the precision with which the rotor  4  is normally held centered by the radial magnetic bearing(s)  7 . This precision with which the rotor  4  is normally held centered is generally very good, being less than a few microns.  
         [0037]    The coaxial stator ring  13  is engaged and strongly braked or prevented from rotating in a housing at the end of the stator  1  between an axial shoulder  22  and a fixing ring  23  held on the stator  1  by screws, with the head of one such screw  24  being visible in the figure.  
         [0038]    In the embodiment shown in FIGS. 3 and 4, the landing bearing comprises rolling elements in the form of spherical balls. These rolling elements comprise an alternating succession of rolling elements having outside surfaces made of steel and rolling elements having an outside surface made of ceramic. Thus, for example, the rolling elements  14   a  and  14   c  have steel outside surfaces, while the rolling elements  14   b  and  14   d  have ceramic outside surfaces.  
         [0039]    For the rolling elements  14   a  and  14   c  made of steel, it is advantageous to use a stainless steel.  
         [0040]    For the ceramic rolling elements  14   b  and  14   d,  it is advantageous to use silicon nitride.  
         [0041]    During landing, the rolling elements  14   a - 14   d  are set into rotation, and adjacent rolling elements such as the elements  14   a  and  14   b  come into contact with each other via portions of their peripheral surfaces, thereby giving rise to friction. Because of the alternation between steel rolling elements and ceramic rolling elements, friction is always produced between two rolling elements that are made of different materials, which encourages slip and thus reduces the friction forces opposing rapid acceleration of the landing bearing.  
         [0042]    The running tracks  20  and  21  (FIG. 2) can be a stainless steel.  
         [0043]    Because only every other rolling element is made of a much expensive material such as ceramic, the extra cost of manufacture is reduced, while simultaneously obtaining very significant advantages in terms of the longevity of the landing bearing and of the vacuum pump in general.  
         [0044]    Another advantage results from the presence of a certain number of rolling elements  14   a,    14   c  being made of steel which is a good conductor of heat, thereby maintaining adequate capacity to cool the rotor. For this purpose, provision is made for the steel rolling elements  14   a,    14   c  to remain in contact with the running tracks  20 ,  21  under normal operating temperature conditions.  
         [0045]    In other words, under such normal operating temperature conditions, the ceramic rolling elements  14   b,    14   d  are of a diameter which is preferably smaller than or not greater than the diameter of the steel rolling elements  14   a,    14   c.    
         [0046]    During landing, operation is as follows: initially, the rotor ring  12  does not touch the shaft  6  which is rotating at high speed about its longitudinal axis I—I. When the radial magnetic bearings such as the bearing  7  cease to operate, the rotor  4  can move radially across the first radial clearance  18  until it makes contact with the rotor ring  12  which is initially stationary and which suddenly finds itself set into rotation, thereby also setting into rotation the rolling elements  14 . The coaxial stator ring  13  is locked or at least braked so far as rotation is concerned within the stator  1 .  
         [0047]    Because of inertia and friction in the landing bearing, the rotor ring  12  does not instantaneously take on the high speed of rotation of the rotor  4 . Rubbing therefore occurs between the bearing surface  17  of the rotor  4  and the corresponding annular inside face  16  of the rotor ring  12 . Because of the reduction in the friction forces that exist between the adjacent rolling elements  14 , the rotor ring  12  can accelerate quickly, thereby reducing the length of time that rubbing occurs between the bearing surface  17  of the rotor  4  and the inside annular face  16  of the rotor ring  12 .  
         [0048]    The present invention is not limited to the embodiments described explicitly above, but covers variant generalizations within the competence of the person skilled in the art. In particular, the landing bearings  9  and  10  could be secured to the rotor  4  instead of being secured to the stator.