Patent Publication Number: US-2011064340-A1

Title: Method and apparatus for stabilizing a squeeze film damper for a rotating machine

Description:
BACKGROUND 
     High speed rotating machines, such as auxiliary power units, may be subject to undesired vibrations during operation. For example, one type of auxiliary power unit may experience relatively high synchronous vibrations at speeds below the operating speed during transitional speed excursions. Such vibrations over time can result in the loss of engine structural integrity, including broken oil tubes, rear bearing turbine assembly failure and damage to the rotor assembly. 
     A squeeze film damper has been used at an interface between a housing and a bearing assembly to dissipate energy associated with “whirling” of the rotor bearing system. The squeeze film damper is intended to reduce rotor vibrations and bearing forces. A whirling condition exists when a rotational axis of the rotor orbits about the intended rotational axis provided by the housing. Despite the damping provided by the squeeze film, the eccentric movement or vibration of the rotor axis about the housing axis can cause damage or failure of rotor bearing system components. 
     SUMMARY 
     A rotor bearing system is disclosed for a rotating machine. The rotor bearing system includes a housing having a bore that provides an inner surface. A bearing assembly is disposed within the bore and includes an outer surface. An annular cavity is provided radially between the outer surface and the inner surface. At least one protrusion extends radially outwardly from at least one of the inner and outer surfaces to an apex and into the annular cavity. A radial gap is arranged between the apex and the opposite surface from which the protrusion extends. In the disclosed example, the annular cavity is filled with an oil to provide a squeeze film damper between the housing and the bearing assembly. The protrusions exert a hydrodynamic preload on the bearing assembly, which reduces vibration during operation of the rotating machine. 
     These and other features of the disclosure can be best understood from the following specification and drawings, the following of which is a brief description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of an example auxiliary power unit. 
         FIG. 2  is an enlarged cross-sectional view of a rotor bearing system illustrated in circle  2  of  FIG. 1 . 
         FIG. 3  is a cross-sectional view taken along line  3 - 3  in  FIG. 2 . 
         FIG. 4  is an enlarged cross-sectional view of a portion of the rotor bearing system with a squeeze film damper with its size exaggerated. 
         FIG. 5  is a partial top elevational view of an example outer cage. 
         FIG. 6  is a partial cross-sectional view of the bearing assembly with a rolling bearing element centered relative to a housing. 
     
    
    
     DETAILED DESCRIPTION 
     An example auxiliary power unit (APU)  10  is illustrated in  FIG. 1 . The APU  10  includes a compressor  12  supported for rotation on a shaft  18 . The axis A 1  is centrally located relative to a housing  22  within which the shaft  18  is supported for rotation by a bearing assembly  20 . A combustor  14  receives compressed air from the compressor  12  and supplies combusted gases to turbines  16 , which rotate the shaft  18 . The shaft  18  may include one or more shaft portions and is rotatable about an axis A 2 . 
     Referring to  FIG. 2 , the bearing assembly  20  includes an outer cage  24  having a radially extending annular flange  25  that is secured to the housing  22  by a fastening element  26 . The outer cage  24  is received within a bore  23  in the housing  22 , which provides the axis A 1 . 
     A rolling bearing element  27  is pressed into the outer cage  24 . In one example, the rolling bearing element  27  is a ball bearing, although other bearings, such as needle bearings, can be used. The rolling bearing element  27  includes rolling elements  32  circumferentially retained by a bearing cage  34  and secured between inner and outer races  28 ,  30 . A retainer  36 , such as a circlip, is used to axially retain the outer race  30  relative to the outer cage  24 . The shaft  18  is received in a press-fit relationship with the inner race  28 . The inner race  28  is axially retained relative on the shaft  18  with a collar  38  that is secured to the shaft  18  by a fastener  40 . 
     Referring to  FIGS. 3-5 , an annular cavity  42  is provided at the interface between the outer cage  24  and the housing  22 . In the example illustrated, the housing  22  receives a liner  41  in a press-fit relationship providing an inner surface  48  that surrounds an outer surface  47  of the outer cage  24 . The outer cage  24  includes annular grooves  45  ( FIG. 5 ) receiving axially spaced apart piston rings  44  ( FIGS. 2 and 4 ) with the outer surface  47  axially arranged between the piston rings  44 . 
     The housing  22 , liner  41 , outer cage  24 , piston rings  44  and outer race  30  are rotationally fixed relative to one another. The shaft  18  and inner race  28  are rotationally fixed relative to one another. 
     The housing  22  provides the axis A 1  about which it is desirable to rotate the shaft  18 . However, due to vibration of the bearing assembly  20  during operation of the APU  10 , the shaft  18  may rotate about the axis A 2  that is offset from the axis A 1 , best shown in  FIG. 3 . The axis A 2  may orbit about the axis A 1  and in part vibration to various components of the APU  10 . Accordingly, a squeeze film  46  is provided in the annular cavity  42  between the inner and outer surfaces  48 ,  47  to damp the eccentric movement of the bearing assembly  20  within the bore  23 . Circumferentially spaced holes  52 , in fluid communication with a fluid source  54 , are provided in the liner  41  to supply a fluid, such as oil, to the annular cavity  42 . 
     Circumferentially spaced lobes or protrusions  50  extend radially inwardly into the annular cavity  42  from at least one of the inner and outer surfaces  48 ,  47 , which are generally cylindrical in shape. Each protrusion  50  is arranged circumferentially between a pair of holes  52 . In the example shown, three protrusions  50  are circumferentially spaced from one another equally and extend from the inner surface  48  to an apex  51 . It should be understood that protrusions may extend from the inner surface  47  instead or additionally. Moreover, more or fewer than three lobes can be used. The apexes  51  do not contact the opposite surface, the inner surface  47  in the example, when the axes A 1 , A 2  are coaxial with one another ( FIG. 6 ). The protrusions  50  create a hydrodynamic preload L ( FIG. 3 ) on the bearing assembly  20 , which damps the eccentric movement of the bearing assembly  20  within the annular cavity  42 . More specifically, the lubricant in the annular cavity  42  is displaced thus creating hydrodynamic pressure that acts on the outer cage  24  (preload L) to damp its relative radial movement between the axes. 
     The bearing assembly  20  is shown centered in the housing  22  in  FIG. 6  such that the axes A 1 , A 2  are coaxial with one another. In this position, a clearance c between the outer surface  47  from which the protrusion  50  does not extend (solid line illustrating the outer surface  48 ) and an area of the inner surface  48  is approximately 0.003-0.005 inch (0.076-0.127 mm) in one example application. The clearance c varies based upon the given application. The height h of the protrusion  50  extends from the outer surface (shown from dashed line) into the annular cavity  42  less than one half the distance of the clearance c. In one example, the height h is less than or equal to 0.3 c. As a result, a radial gap (c-h) is provided between the apexes  51  (only one shown in  FIG. 6 ) and the inner surface  47  with the axes A 1 , A 2  coaxial with one another. 
     Although example embodiments have been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of the claims. For that reason, the following claims should be studied to determine their true scope and content.