Patent Publication Number: US-2012045154-A1

Title: Multiblade Gasodynamic Bearing

Description:
TECHNICAL FIELD 
     The invention is related to mechanical engineering in particular to compliant foil hydrodynamic bearings which is used in small-size high speed machines such as turbocompressors, cooling turbines etc. 
     BACKGROUND ART 
     The known foil hydrodynamic bearing (U.S. Pat. No. 5,634,723) comprises the bearing case, the journal, disposed inside the bearing case, several compliant smooth top foils, disposed in the circumferential direction in the space between the inner cylindrical bearing case surface and the journal. The spring element in form of corrugated foil is disposed between the inner bearing case surface and each top foil. The compliant smooth inner foil is disposed between each top foil outer surface and the corresponding spring element inner surface. Each inner foil is fixed by one end thereof to the top foil that is disposed next to said inner foil in the circumferential direction. 
     The bearing load capacity of some foil hydrodynamic bearing increases under other constant factors when the angle length of gas lubricating layer, disposed between each top foil inner surface and the journal surface increases. It means that it is necessary to reduce the quantity of top foils in the bearing in order to increase said bearing load capacity. 
     At small rotation speed the known foil hydrodynamic bearing (U.S. Pat. No. 5,634,723) has reduced damping capacity in case of small quantity of top foils, for example equal to three. The reason is that at radial journal oscillations in the direction of some top foil there is no sliding between the top foil and inner foil disposed radially outwardly of said top foil. Said sliding does not appear because part of said top foil is in the zone where there are big thickness and small over pressure in the lubricating layer. While one part of said top foil with small thickness of lubricating layer radially displaces from the bearing centre, another part of said top foil with big thickness of lubricating layer straitens up and expands without sliding between the top and inner foil. 
     Reduced damping capacity of said foil hydrodynamic radial bearing at small journal rotation speed is a disadvantage because resonant rotor oscillations arise in bearings at start/stop. Low damping causes an increase in magnitude of resonant rotor oscillations and leads to necessity to increase radial gaps between stationary parts and compressor wheel or turbine wheel that decreases high speed machines efficiency. 
     SUMMARY OF THE INVENTION 
     The object of the presented invention is to increase damping capacity of foil hydrodynamic radial bearing at small rotor revolution speed. 
     The appointed object is achieved by the following way. The foil hydrodynamic radial bearing comprises the bearing case with journal, two or more compliant smooth top foils.. 
     Said top foils are disposed in the annular space between the bearing case inner surface and the journal and they extend around the journal. Said top foils adjoin by their inner surface to the journal. One end of the top foil is fixed in the direction of bearing axis to the bearing case. Between each top foil outer surface and the bearing case inner surface there are disposed in the circumferential direction two or more elastically damping sections. Each of said sections comprises the spring element (for example, a corrugated foil) adjoined by its outer side to the bearing case and two or more compliant smooth inner foils disposed between the spring element inner surface and the top foil outer surface. One end of inner foil is fixed in direction of the bearing axis to the bearing case. Between each top foil and the bearing case there is disposed at least one elastically damping section wherein two contacting inner foils are fixed to the bearing case near opposite ends of the spring element. 
    
    
     DESCRIPTION OF THE EMBODIMENT 
     Embodiment of the present invention is explained below by reference to the attached drawing. 
       FIG. 1  illustrates the cross sectional view of the radial foil hydrodynamic bearing. 
     The foil hydrodynamic radial bearing comprises the rotor journal  1  disposed inside the hole of the bearing case  7 . Compliant smooth top foils  15  adjoined by inner surfaces  20  to the rotor journal  1  are disposed in annular space between the inner surface  5  of the bearing case  7  and the surface  10  of the journal  1  The end  17  of top foil is fixed in the direction of bearing axis to the bearing case, for example by welding. The top foil extends around the journal. The unfixed end of said top foil forms a small gap with the fixed part of next top foil. 
     Several (at least two) elastically damping sections are disposed in the circumferential direction between the outer surface  22  of each top foil and the bearing case inner surface. The bearing shown in  FIG. 1  has two such sections under each top foil. Each elastically damping section comprises the spring element (for example, a elastic corrugated foil)  25  and compliant smooth inner foils  27 ,  30 ,  33 . The inner foil  27  contacts by its outer surface with the spring element inner surface. The inner foil  30  contacts by its outer surface with the inner surface of inner foil  27 . The inner foil  33  contacts by its outer surface with the inner surface of inner foil  30 . The number of inner foils in the elastically damping section may be equal to two or more. The inner foils  27 ,  30  and  33  are fixed near spring element  25  to the bearing case along one end in the direction of bearing axis. One of the manners to fix inner foils is welding. The inner foils  27  and  30  are directly fixed by parts  35  and  40  to the bearing case. If number of inner foils in the section more than two, part of inner foils may be fixed to the bearing case by fixing parts of underlying inner foils. For example, overlying inner foil  33  is fixed by its part  37  to the bearing case thorough fixing part  35  of the underlying inner foil  27 .  FIG. 1  shows one of possible variants to dispose inner foils fixing parts in the section wherein each pair of contacting inner foils (pair of foils  27     30 , pair of foils  30     33 ) is fixed to the bearing case near opposite ends of the spring element. 
     In the operation of the foil hydrodynamic bearing-according to the embodiment of the present invention, rotating journal surface  10  entrains in the circumferential direction the air in lubricating layers between top foils inner surfaces  20  and the journal surface  10 , that is from inlet to outlet disposed at free end and fixed end  17  of each top foil correspondently. 
     For the top foil  22  disposed in the direction of bearing load it correspondents to air movement in said layer from its big to small thickness. For said top foil air pressure in lubricating layer increases with decrease in layer thickness by reason of air viscosity. At some rotation speed the value of pressure becomes sufficient to prevent contact between the journal  1  and the top foils inner surfaces  20 . 
       FIG. 1  shows the variant of disposing bearing where bearing load under rotor weight passes to the low part of bearing case in the zone of small thickness in lubricating layer. At small rotation speed, big lubricating layer overpressure is only in said zone and main part of bearing load is passed to the bearing case through the top foil disposed in low part of the bearing and the low elastically damping section: inner foils  33 ,  30 ,  27  and the spring element  25 . 
     Arising oscillations of the rotating shaft in the foil hydrodynamic radial bearing are accompanied with frictional damping because of sliding bearing elements relatively each other: top and inner foils, spring elements and the bearing case. 
     At vertical oscillations of the shaft and small rotation speed the main part of frictional damping is generated at the bearing low part where contact pressure between bearing elements is maximal. 
     When the journal moves down, the top foil and elastically damping section inner foils displace down under lubricating layer pressure, inner and outer surfaces of the inner foil  30  displace relatively the bearing case in a clockwise direction to fixing part of inner foil  30 . Surfaces of inner foils  33  and  27  displace in a counterclockwise direction. Said displacement of contacting inner foils in different directions generates friction forces between inner foils  33  and  30  and between inner foils  30  and  27 . Angle length of elastically damping sections is so that practically all low part of said section is disposed in high pressure zone and small thickness of lubricating layer. That is why under friction forces the inner foils in said section cannot approach to the journal and straighten, so they have to slide relatively each other generating frictional damping. When the journal moves up, that is in backward direction, inner foils of said section return to the initial place and also slide relatively each other generating frictional damping. 
     When the shaft oscillates in another direction or in case of shaft circular precession, by the same way, as a result of journal movement, there arises friction damping in other deforming elastically damping sections. 
     Friction damping between inner foils of elastically damping section increases at increase in number of inner foils contacting couples. If the elastically damping section has only two inner foils, there is one inner foil contacting couple. If the elastically damping section has three inner foils, as it is shown in  FIG. 1 , there are two inner foils contacting couples and in this case the friction damping will be more than at two inner foils in the section.