Abstract:
A turbocharger rotor includes a turbine wheel, a compressor wheel, a shaft extending between the turbine and compressor wheels for rotation together about an axis, and connecting means. The connecting means include first and second joints including alignment couplings joining opposite ends of the shaft with adjoining inner ends of the compressor wheel and the turbine wheel. The couplings are configured to coaxially align and drivingly engage the shaft with the compressor and turbine wheels. A fastener rod extends through the shaft and the compressor wheel, engaging the turbine wheel to retain the rotor components together under compressive load. The rod is resiliently stretchable to limit changes in the retaining force changes in axial dimensions during operating and stationary conditions. Additional features and variations are disclosed.

Description:
TECHNICAL FIELD 
     This invention relates to engine exhaust driven turbochargers and more particularly to a turbocharger rotor having alignment couplings and a fastener rod joining compressor and turbine wheels with a connecting shaft. 
     BACKGROUND OF THE INVENTION 
     It is known in the art relating to exhaust driven engine turbochargers to provide a rotor including a turbine wheel and a compressor wheel connected by a shaft for rotation together about an axis. In some cases, the shaft is formed as an extension of the turbine wheel. Separate shaft and wheel components may be welded together before final machining. Alternatively, a steel shaft may be connected to the turbine and to the compressor wheel by separate connecting means. Commonly, the impeller or compressor wheel is made of aluminum alloy to minimize the rotating mass. 
     Various types of connecting means have been provided for aligning and connecting the wheels and the shaft for axial rotation. Where the connecting means extend through the compressor wheel and clamp the wheel in compression against the shaft, the design should avoid excessive variations in clamping load due to differential thermal growth and the effects of centrifugal force on the steel and aluminum during varying operating and stationary conditions. The means for connecting the compressor impeller wheel and the turbine wheel to the shaft are also important because the rotor must be disassembled after balancing in order to assemble the rotor into the turbocharger. Upon reassembly of the rotor, the repeat balance must preserve the original balance as far as possible without actually rebalancing the rotor in the turbocharger assembly. Connecting means that allow separation and reassembly of the components without changing the balance are therefore desired. 
     SUMMARY OF THE INVENTION 
     The present invention provides a rotor including a turbine wheel and a compressor wheel connected by a shaft for rotation together about an axis. Novel connecting means extend between the compressor and turbine wheels and limit the clamp load, or retaining force, variation applied to the compressor wheel under varying thermal expansion conditions experienced during operation and shutdown. The connecting means also provide for coaxially aligning or centering the compressor and turbine wheels on the axis of the connecting shaft with the capability of simple and repeatable reassembly. 
     The connecting means include a single long fastener rod, such as a stud or bolt, which extends through both the compressor wheel and the connecting shaft to engage the turbine wheel and place both the compressor wheel and the connecting shaft in compression. Preferably the fastener rod is threaded into the turbine wheel and carries a nut or head that clamps the compressor wheel and shaft in assembly with the turbine wheel. Optionally, the fastener rod could also extend through the turbine wheel and be secured to the turbine wheel by a nut or head. 
     The connecting means also include first and second joints between the shaft and the compressor wheel at one end and the turbine wheel at the other end. The joints are configured to maintain coaxial alignment of the compressor and turbine wheels with the shaft while providing high axial and bending stiffness and torque transmitting capability. Various forms of joints could be provided to meet these requirements. Examples include piloted shoulders and polygon connections as well as toothed couplings, among others. A presently preferred embodiment uses toothed couplings with so-called CURVIC™ coupling teeth. 
     Another preferred feature of the invention includes use of a steel adapter which is press fitted onto a stub of the aluminum alloy compressor wheel to provide a joint material similar to that of the connecting shaft. The adapter may also provide an oil sealing surface. A similar adapter may also be provided on the turbine wheel if desired. 
     The shaft may include one or more radial thrust surfaces preferably located inboard of associated bearing journals to limit oil sealing requirements. The thrust surfaces preferably face outward and are formed on flanges integral with the shaft. 
     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 
     In the drawings: 
     FIG. 1 is a cross-sectional view of an engine turbocharger having a rotor including features in accordance with the invention; 
     FIG. 2 is a side view partially in cross section of the rotor in the embodiment of FIG. 1; 
     FIG. 3 is an end view from the plane of the line  3 — 3  of FIG. 2 showing a toothed coupling portion of the compressor wheel; 
     FIG. 4 is an enlarged end view of the compressor wheel coupling teeth shown in the circle  4  of FIG. 3; 
     FIG. 5 is an enlarged end view of the rotor shaft coupling teeth configured for mating with the compressor wheel coupling teeth; and 
     FIG. 6 is a view similar to FIG. 2 but showing a modified embodiment of the invention; 
     FIG. 7 is a fragmentary cross-sectional view showing an alternative rotor having an exemplary piloted shoulder coupling; 
     FIG. 8 is a view similar to FIG. 7 but showing a polygon coupling; and 
     FIG. 9 is an end view from line  9 — 9  of FIG. 8 showing the shape of the polygon recess in the shaft coupling. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     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 . 
     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 the 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 . 
     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 bearings  48 ,  50  that rotatably support the rotor  12 . Bearing  48  is a combination sleeve and thrust bearing while bearing  50  is primarily a sleeve bearing. 
     Referring particularly to FIG. 2, the rotor  12  includes a shaft  52  connected with the turbine wheel  18  at one end and the compressor wheel  20  at the opposite end. The shaft  52  includes a pair of axially spaced bearing supported portions or journals  54 ,  56 , respectively adjacent the compressor and turbine wheel ends of the shaft. A flange  57 , inboard of journal  54 , carries a radial thrust reaction surface  58 . A second flange  59 , inboard of journal  56 , carries a radial anti-thrust reaction surface  60 . Journals  54 ,  56  are respectively supported in bearings  48 ,  50  (FIG.  1 ). Radial surface  58  carries thrust forces to the sleeve/thrust bearing  48  and radial surface  60  limits axial movement of the rotor  12 . 
     A particular advantage of the invention is gained by having the thrust reaction surface  58  and the anti-thrust reaction surface  60  both face outward toward the ends of the shaft  52 . This is made possible by separating the shaft from the compressor and turbine wheels and allows both flanges  57 ,  59  to be made integral with the shaft, which avoids separate thrust flanges and simplifies machining of the shaft itself. The separation also benefits design modification and rebuild functions because modification or replacement of the turbine or compressor portions need not affect the bearings or the shaft portion. 
     In accordance with the invention, the rotor elements including the compressor wheel  20 , shaft  52  and turbine wheel  18  are retained in assembly by connecting means including a fastener rod, preferably comprising a stud  62  and nut  64 . The stud  62  extends through axial openings in the compressor wheel  20  and the shaft  52  and is threaded into a threaded recess in an inner end  66  of the turbine wheel  18 . The nut  64  is threaded onto an opposite end of the stud and engages a washer  68  on an outer end of the compressor wheel. The nut  64  is tightened a predetermined amount to place under compressive load additional elements of the connecting means, including connections or first and second joints  70 ,  72  between the shaft  52  and the compressor wheel  20  and turbine wheel  18  respectively. 
     The stud  62  is sized to resiliently stretch a desired amount as the nut is tightened to compress the rotor elements. In this way, variations in the compressive force on the rotor elements due to axial dimensional changes in the rotor components, in operation or while stationary, are limited by stretching of the stud  62  so that excessive variations in compressive load are not encountered. This is particularly desirable, since the compressor wheel is made of aluminum alloy, which has a greater thermal coefficient of expansion than the stud  62  and other elements of the rotor made of steel. If desired, another suitable form of fastener rod, such as a long bolt with a head, could be used in place of the stud  62  and nut  64 , as long as the force limiting feature of the fastener rod is retained. Use of a fastener rod to load and connect the rotor elements axially requires only a relatively small axial opening through the compressor wheel and a small threaded recess in the turbine wheel. Thus, stresses in the wheels are reduced as compared to other connecting methods and increased maximum rotor speeds are permitted. 
     In accordance with the invention, the first and second joints  70 ,  72  of the connecting means are provided for aligning and connecting the compressor and turbine wheels on their respective ends of the shaft  52 . The joints  70 ,  72  must maintain coaxial alignment of the compressor and turbine wheels with the shaft while providing high axial stiffness under compression, high bending stiffness, and torque transmitting capability. Many joint configurations exist that could meet the above requirements and are intended to be included within the broad scope of the invention. Accuracy, reliability and cost are also factors to be considered in selecting a suitable joint configuration. 
     Presently preferred embodiments of joints  70 ,  72  are illustrated in FIGS. 2-5. The compressor wheel  20  includes on an inner end a stub  74  carrying a pressed-on steel adapter  76  having a ring shaped end face  78  of the compressor wheel that engages a compressor end  80  of the shaft  52  at the first joint  70 . Adapter  76  also includes a generally cylindrical seal surface  81 , for cooperating with a compressor oil seal of the turbocharger to control oil leakage toward the compressor wheel  20 . The turbine wheel  18  similarly includes on its inner end  66  a steel adapter  82  having a ring shaped end face  84  that engages a turbine end  86  of the shaft  52  at the second joint  72 . Adapter  82  also includes a generally cylindrical seal surface  87  for cooperating with a turbine oil seal to control oil leakage toward the turbine. The inboard location of the thrust flanges and their reaction surfaces  58 ,  60  of shaft  52  also helps control oil seal leakage, because oil flowing from the thrust flanges is directed away form the oil seal surfaces  81 ,  87 . 
     FIGS. 3-5 show details of the first joint, which are similar to those of the second joint. The end face  78  of the compressor wheel  20  mounts an axially centered first ring of coupling teeth  88  extending axially inward from the end face  78  toward the compressor end  80  of the shaft  52 . The shaft  52  similarly has on the compressor end  80  a second ring of mating coupling teeth  90  extending axially outward into engagement with coupling teeth  88  of the first ring. Preferably, the coupling teeth take the form of a so-called CURVIC™ coupling in which the first ring of teeth  88  of the compressor wheel are formed with concave sides separated by convexly sided spaces  92  and the mating teeth  90  on the shaft have convex sides separated by concavely curved spaces  94 . These configurations are best shown in FIGS. 4 and 5. 
     The second joint  72  similarly includes an axially centered third ring of coupling teeth  88  extending axially inward from the end face  84  of the turbine toward the turbine end  86  of the shaft  52 . The shaft similarly has on the turbine end  86  a fourth ring of mating coupling teeth  90  extending axially outward into engagement with coupling teeth  88  of the third ring. These teeth also preferably take the form of a CURVIC™ coupling as described above. The toothed couplings at the first and second joints meet the requirements of the joints by maintaining coaxial alignment of the compressor and turbine wheels with the shaft while providing high axial stiffness when under compression with high bending stiffness, and torque transmitting capability. 
     The rotor  12  is first assembled outside the turbocharger as shown in FIG.  2 . It is balanced, marked to show the locations of the mating coupling teeth and subsequently disassembled for reassembly with other components in the buildup of a complete turbocharger. Upon reassembly within the turbocharger, the rotor components are axially aligned by the toothed couplings and angularly positioned with the same phase angles maintained during balancing by aligning the marked teeth of the couplings. The reassembled rotor is thus maintained in essentially the same balance condition as originally provided by the original balance operation outside of the turbocharger. 
     Referring now to FIG. 6 of the drawings wherein like numerals indicate like parts or features, numeral  100  indicates a turbocharger rotor similar to that of FIG.  2 . Rotor  100  differs from rotor  12  in that the turbine adapter is replaced by a seal collar  102 , which forms a cylindrical seal surface  104  but does not form an inner face of the turbine wheel  106 . Instead, a stub  108  of the wheel  106  has an inner end  110  integral with a ring shaped inner face  112  and a third ring of coupling teeth  114  integrally formed on the inner face  112 . Teeth  114  may be configured like teeth  88  on the turbine wheel adapter  82  of the embodiment of FIG. 2, and so the turbine wheel  106  may be made interchangeable with turbine wheel  18  illustrated in FIGS. 1 and 2. The coupling teeth may be formed on the turbine wheel because the turbine wheel material has a hardness similar to the shaft  52  to which it is coupled. The aluminum material of the compressor wheel makes use of the adapter  76  necessary, or at least desirable, to avoid having aluminum teeth on the compressor wheel  20  engaging steel teeth on the shaft  52 . 
     FIGS. 7-9 illustrate two examples of alternative joint configurations that could be selected for use in a turbocharger rotor of according to the invention. These examples are not meant to limit the scope of the invention, but only to show some considered alternatives. 
     FIG. 7 illustrates one form of piloted shoulder coupling joint  116  located at the inner end of compressor wheel  20  but also usable at the joint between the shaft and turbine wheel, not shown. Joint  116  includes a male coupling  118  formed on an adapter  120  fixed on the inner end of the compressor wheel  20 . Coupling  118  includes an annular shoulder  122  surrounding a protruding cylindrical pilot  124  formed with a circular cross section. A mating female coupling  126  is formed in an end of the connecting shaft  128  and includes an annular abutment  130  engaging the shoulder  122 . A cylindrical recess  132  is axially centered on the shaft end and receives the pilot  124  of coupling  118  with a close fit. The pilot  124  and surrounding shoulder  122  and the mating recess  132  and abutment  130  of the couplings assure coaxial alignment of the compressor wheel  20  with the shaft  128  when the components are compressed by the stud  62  and nut  64  comprising the fastener rod. A similar coupling joint, not shown, may be applied at the turbine end of the shaft  128 . Preferably, a dowel  134  connects the adapter  120  with the shaft  128  to maintain angular positioning of the components upon reassembly of the rotor. 
     FIGS. 8 and 9 illustrate one form of so-called polygon coupling joint  136 . The polygon joint is similar to the piloted shoulder joint  116  just described and may be used in the same locations. The adapter located polygon coupling  138  differs in that the protruding pilot  140  and the mating recess  142  of the shaft coupling  144  of shaft  146  have polygon shaped cross sections as shown, for example, by recess  142  in FIG.  9 . The shoulder  148  of the male coupling  138  and the mating abutment  150  of the shaft coupling  144  differ in configuration but have the same purpose as the similar features  122 ,  130  of joint  116 . With the polygon joint  136 , a locating dowel is not needed, since marking the assembled rotor components allows reassembly in the same location determined by the polygon pilot. In other ways, coupling joints  136  and  116  may be essentially the same. 
     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 fall scope permitted by the language of the following claims.