Abstract:
An improved turbocharger for railway locomotive sized engines includes, in a preferred embodiment, first and second axially spaced ball bearings supporting a rotor with the first bearing being a hybrid ceramic ball bearing mounted to accept both radial and axial loads acting on the shaft at the compressor end. The first bearing is mounted on a reduced diameter portion of the shaft, providing reduced bearing diameter to acceptably limit centrifugal loading of ceramic balls in the bearing against a surrounding bearing race. The first bearing has dual rows of ceramic ball bearings mounted to share all axial thrust loads on the shaft. The second bearing is also a ball bearing. Lubrication of the bearings is preferably by direct impingement on the inner race to minimize oil churning causing heating and power loss. Additional features and advantages are disclosed.

Description:
TECHNICAL FIELD  
         [0001]    This invention relates to engine turbochargers and particularly to a novel ball bearing mounting of a high thrust turbocharger rotor.  
         BACKGROUND OF THE INVENTION  
         [0002]    A turbocharger for a medium speed diesel engine, adaptable for use in railway road locomotives and other applications, has a rotor with a radial flow compressor wheel or impeller and an axial flow turbine wheel or turbine, unlike typical automotive turbochargers. The wheels are carried at opposite ends of a connecting shaft supported at two spaced bearing locations with the wheels overhung. This configuration is known as a flexible rotor, since it will operate above its first, and possibly second, critical speeds. It can therefore be subject to rotor dynamic conditions such as whirl and synchronous vibration.  
           [0003]    High thrust loads are created by the difference in air pressures across the turbine and compressor wheels. These loads can be quite large due to the relatively large radial area of the wheels. The net thrust loads on the wheels are in the same direction, creating a high overall thrust on the rotor. The radial load due to the static weight of the rotor is comparatively small.  
           [0004]    Turbocharger design can include the use of sealing devices at the rim of turbine wheel to help control pressure on the face of the turbine wheel inboard of the blades. This is feasible because the high temperature turbine end materials have more closely matched thermal expansion coefficients than the aluminum wheel and ferrous housing materials typical of the compressor end of the turbocharger. Thus, at the turbine end, a reasonable range of clearances can be obtained.  
           [0005]    On the upstream end, the aim is to keep the flowpath pressure off the face of the turbine wheel. This pressure pushes in the same direction as the thrust on the compressor wheel. On the downstream end, if the face could be pressurized it would help to reduce the compressor wheel thrust effect by pushing the other way. In practice, this is difficult, because the seal must be made very tight or else an extremely high flow of pressurized air is required, only to be directly exhausted out of the turbocharger without being used to do any work.  
           [0006]    Diesel locomotive engines, and turbochargers, may operate over an extremely large range of conditions, from minus 40 degrees at startup to the high temperatures and high turbine speeds experienced in a high altitude tunnel. With aluminum compressor wheels chosen for low inertia and quick response, their rotating and static thermal coefficients are poorly matched to the housing so that sealing the back face of the compressor wheel is not a currently practical option. Since the compressor pressure ratio is considerably higher than that of the turbine, a higher pressure acts over an area about equal to that of the turbine.  
           [0007]    Even with the use of seals where practical, and more so without them, the high thrust loads acting on the rotor, as well as the potential for whirl and vibration, have made hydrodynamic fluid film bearings the universal choice for turbochargers of this type as compared to the common use of ball bearings in automobile engine turbochargers. Hydrodynamic fluid film bearings feature high load capacity, variable stiffness, essentially infinite life if the fluid film is maintained, and allow large shaft diameter for better stiffness and lower vibration. However, they require high oil flow and cause high power losses, which reduce overall efficiency.  
           [0008]    Ball bearings require much lower oil flow and operate with lower power loss for improved efficiency as well as more consistent stiffness over the operating range. However, they have lower thrust load capacity, have finite operating life due to metal fatigue of the moving parts, and must be limited in diameter so that high rotating speeds do not put excessive centrifugal loads on the balls. As a result, ball bearings are not known to have been successfully applied to turbochargers of the type described as used in railroad engines and other applications.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention provides a turbocharger, adapted for use in railroad locomotive engines and other applications, combined with a ball bearing rotor mounting capable of accepting both radial support loads and axial thrust loads applied to the rotor of a railroad engine turbocharger.  
           [0010]    In a preferred embodiment, the turbocharger includes a housing carrying a rotor having an axial flow turbine wheel and a radial flow compressor wheel. The wheels are supported at opposite ends of a shaft carried in the housing on oil lubricated first and second bearings spaced axially adjacent to compressor and turbine ends respectively of the shaft. The arrangement provides an overhung rotor mounting with axial thrust loading normally applied to the shaft from both wheels in the same direction from the turbine toward the compressor.  
           [0011]    In the improved assembly, the first bearing includes at least one hybrid ceramic ball bearing mounted to accept both radial and axial loads acting on the shaft at the compressor end. The first bearing is mounted on a reduced diameter portion of the shaft, providing reduced bearing diameter to acceptably limit centrifugal loading of ceramic balls in the bearing against a surrounding bearing race.  
           [0012]    Additional features may include a first bearing having dual rows of ceramic ball bearings mounted to share all axial thrust loads on the shaft. The second bearing may also be a ball bearing and, optionally, a hybrid ceramic thrust bearing on a reduced diameter shaft portion to limit centrifugal loading of the balls in the bearing. Lubrication of the bearings is preferably by direct impingement on the inner race to minimize oil churning causing heating and power loss. The shaft between the bearings preferably has a greater diameter than at the bearing locations to maintain adequate bending stiffness in the overhung rotor. The second bearing may be made slidable in the housing to direct all thrust loads to the dual row first bearing. A squeeze film damper may carry the second bearing to minimize whirl at the turbine end of the rotor. The shaft may be separate from the compressor and turbine wheels and include a yieldable fastener, such as a stud or bolt extending through the compressor wheel and the shaft to engage the turbine wheel and maintain a relatively constant clamping load on the rotor.  
           [0013]    These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    In the drawings:  
         [0015]    [0015]FIG. 1 is a cross-sectional view of an engine turbocharger having a ball bearing mounted rotor according to the invention;  
         [0016]    [0016]FIG. 2 is an enlarged cross-sectional view of the rotor and bearing mounting portions of the turbocharger of FIG. 1; and  
         [0017]    [0017]FIG. 3 is a cross-sectional view through the rotor shaft toward the end of compressor adapter showing a polygon drive connection.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]    Referring now to the drawings in detail, numeral  10  generally indicates an exhaust driven turbocharger for an engine, such as a diesel engine intended for use in railway locomotives or other applications of medium speed diesel engines. Turbocharger  10  includes a rotor  12  carried by a rotor support  14  for rotation on a longitudinal axis  16  and including a turbine wheel  18  and a compressor wheel  20 . The compressor wheel is enclosed by a compressor housing assembly  22  including components which are supported on an axially facing first side  24  of the rotor support  14 . An exhaust duct  26  has a compressor end  28  that is mounted on a second side  30  of the rotor support  14  spaced axially from the first side  24 .  
         [0019]    The exhaust duct  26  is physically positioned between the rotor support  14  and the turbine wheel  18  to receive exhaust gases passing through the turbine wheel and carry them to an exhaust outlet  32 . A turbine end  34  of the exhaust duct  26  and an associated nozzle retainer assembly  35  are separately supported by an exhaust duct support  36  that is connected with the exhaust duct  26  at the turbine end  34 . The exhaust duct support  36  also supports a turbine inlet scroll  38  which receives exhaust gas from an associated engine and directs it through a nozzle ring  40  to the turbine wheel  18  for transferring energy to drive the turbocharger compressor wheel  20 .  
         [0020]    The rotor support  14  includes a pair of laterally spaced mounting feet  42  which are rigidly connected to an upstanding mounting portion  44  of the rotor support  14  and are adapted to be mounted on a rigid base, not shown. The rotor support  14  further includes a tapering rotor support portion  46  having ball bearings  48 ,  50  that rotatably support the rotor  12  and are subsequently further described.  
         [0021]    Referring particularly to FIG. 2, the rotor  12  includes a shaft  52  extending between and operatively engaging inner ends of the turbine wheel  18  and the compressor wheel  20 . A resilient fastener in the form of a stud  54  extends through axial openings in the compressor wheel  20  and shaft  52  and engages a threaded opening in the turbine wheel  18 . A nut  56  on the stud  54  engages a washer on the outer end of the compressor wheel to clamp the rotor components together with a desired preload. The stud is resiliently stretched so that the preload remains relatively constant in spite of variations in the axial length of the rotor assembly under operating and stationary conditions.  
         [0022]    In accordance with the invention, the rotor  12  is supported by first and second axially spaced ball bearings  48 ,  50 , respectively. The bearings engage reduced diameter mounting portions  58 ,  60  at opposite ends of the shaft  52 . The mounting portion diameters are sized to reduce the bearing race diameters to maintain centrifugal forces on the bearing balls within acceptable limits. The portions of shaft  52  between the mounting portions are maintained large to provide a stiff connection between the compressor and turbine wheels.  
         [0023]    At the compressor end of the shaft, bearing  48  includes dual rows of hybrid ceramic ball bearings having inner and outer races  62 ,  64  in axial engagement for transferring thrust loads. The inner races  62  are clamped by a nut  66  against a shoulder  68  at the inner end of the mounting portion  58 . The outer races  64  are received in a bore of a bearing housing  70  that is secured in and radially located by the rotor support portion  46  of the rotor support  14 . The dual row bearing  48  transfers primary thrust loads to a radial flange  72  of the bearing housing  70 . A retainer plate  73  mounted on the bearing housing  70  traps the outer races  64  in the bearing housing and limits axial motion during axial thrust reversals.  
         [0024]    An oil feed passage  74  in the bearing housing sprays oil directly from the flange  72  into the bearing  48  between the inner and outer races. Excess oil from the bearing drains in part through a drain passage  76  into an open drain area  78 . An oil seal member  80  is radially located by the bearing housing  70  but is axially located by mounting to the rotor support portion  46 . Member  80  cooperates with a seal adapter  82  fixed on a stub of the compressor wheel  20  to limit oil leakage from the bearing  48  toward the compressor wheel.  
         [0025]    At the turbine end of the shaft  52 , bearing  50  is a single row bearing having inner and outer races  84 ,  86 . The bearing  50  may be a conventional or hybrid ceramic type and can be made smaller as it carries primarily relatively light radial loads. The inner race  84  is secured by a nut  88  against a shoulder  90  at the inner end of the reduced diameter mounting portion  60 . The outer race  86  is carried in a squeeze film damper (SFD) sleeve  92  that floats in a SFD housing  94  fixed in the rotor support portion  46 . Oil is supplied to the SFD through a groove in the SFD housing which also supplies an oil feed passage  96  that delivers oil directly to the bearing balls between the races  84 ,  86 . A preload spring stack  98 , between a flange of the SFD housing  94  and the SFD sleeve  92 , biases the sleeve and the bearing outer race  86  axially toward the shoulder  90  to maintain continuous axial load on the balls during limited axial bearing motion and avoid ball skidding and subsequent fatigue.  
         [0026]    An adapter  100  on a stub of the turbine wheel cooperates with a seal member  102  mounted on the rotor support portion  46  to limit oil leakage toward the turbine wheel. The bearing  50  is drained directly into the central oil drain area  78  of the rotor support portion  46 .  
         [0027]    Ball bearings used in high speed rotating machines tend to be life limited by centrifugal forces acting on the bearing balls. The size of the bearing balls and the diameter of the ball races are thus important factors in the application of ball bearings to turbomachinery. Accordingly, ball bearings are commonly used with small automotive engine turbocharger rotors because the small diameters of balls and races permit long life with conventional bearing materials. For the same reasons, ball bearing applications are not found in large engine turbochargers with large diameter shafts and heavy thrust loads requiring larger bearings.  
         [0028]    The present invention overcomes these problems by combining several features that make the application of ball bearings practical in engines of a size useful in diesel road freight locomotives and other comparable applications. For example, at least the larger, thrust carrying bearing  48  is mounted on a reduced diameter portion  58  at the end of the shaft. This allows the bearing race diameter to be reduced while the portion of the shaft between the bearings remains large as is needed for adequate stiffness. A double row bearing is used if needed to carry the high thrust loads involved. Also, hybrid ceramic ball bearings are used in, at least, the high load position. The ceramic balls are lighter than alloy steel but have high capacity so that the centrifugal force of the balls is reduced and the fatigue life of the bearings is extended.  
         [0029]    Because the diameters of the shaft ends are reduced, a more compact and efficient drive coupling is provided between the shaft  52  and the turbine and compressor wheels  18 ,  20 . P 3  polygon shaped openings  104  are presently preferred to couple the shaft  52  to mating polygon projections  106  extending from adapters  82 ,  100 , which are pressed onto the wheels  18 ,  20  and provide running lands or surfaces for the labyrinth seals. FIG. 3 is a cross-sectional view through the shaft  52  toward the end of the adapter  82  showing the shape of the preferred P 3  polygon projection  106  which mates with a similarly shaped polygon opening  104  in the adapter  82 . If desired the projections could extend from the shaft and mate with openings formed in the adapters.  
         [0030]    In operation of the turbocharger  10 , pressurized exhaust gas is delivered through the turbine inlet scroll  38  to the turbine wheel  18  where it imparts energy to the turbine blades to drive the rotor  12  and is then exhausted at a lower pressure. Higher gas pressure on the inlet face of the turbine wheel yields an axial thrust force in the direction of the compressor wheel. The rotating compressor wheel  20  draws in ambient air moving axially and exhausts it radially at a higher pressure to the compressor housing  22 . The outlet pressure acts against the inner side of the compressor wheel  20  and yields an additional axial thrust force on the rotor, adding to the thrust of the turbine wheel  18 . These thrust forces are absorbed fully by the dual row ceramic ball bearing  48  which carries the thrust loads from the turbine shaft  52  to the bearing housing  70  and thus to the rotor support  14 .  
         [0031]    The thrust loads generate forces much higher than the radial support loads, which are shared between the rotor bearings  48  and  50 . Bearing  50  is allowed to move axially with its squeeze film damper (SFD) sleeve  92  in the SFD housing  94 . However, it is expected to handle small transient thrust loads opposite to the direction of primary thrust forces. The spring stack  98  biases the bearing outer race toward the shaft shoulder  90  to maintain a small axial load on the bearing balls. Thus, bearing  50  carries primarily radial loads and may be made smaller than bearing  48 . The axial loading of bearing  50  helps to avoid ball skidding which could adversely impact bearing fatigue life. The squeeze film damper is applied to counteract so called shaft whirl where the shaft or turbine wheel tends to orbit if the bearing is too lightly loaded. However a squeeze film damper may not be required in all turbocharger applications.  
         [0032]    Lubrication of the bearings by direct impingement of oil at the ball/race interface together with limiting the amount of oil delivered and draining excess oil quickly, avoids oil churning, bearing overheating and failure. The power losses from pumping the oil and viscous resistance of prior hydrodynamic bearings are greatly reduced with the ball bearings and oil delivery system of the disclosed embodiment.  
         [0033]    Advantages of the present invention over turbochargers using the current bearing technology include, without limitation, reduced oil consumption and horsepower loss without the need for expensive dynamic air seals, improved rotor dynamics with the use of bearings more appropriate for the relatively light radial loads, simplified shaft seals due to the low oil consumption, and a potentially less complex oil supply system for the turbocharger.  
         [0034]    While the invention has been described by reference to certain preferred embodiments, it should be understood that numerous changes could be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the disclosed embodiments, but that it have the full scope permitted by the language of the following claims.