Patent Abstract:
A turboalternator system includes a turboalternator having a rotatable member operatively engaged to a bearing set, a radial support element, and a contact structure engaged with the radial support element. The rotatable member defines a first end, a second end and an axis of rotation. The turboalternator system is configured to be thermally adjustable such that in a first thermal condition the contact structure is at a first radial position with respect to the axis of rotation and contacts the rotatable member to provide support, and in a second thermal condition the contact structure is at a second radial position with respect to the axis of rotation that is spaced further from the axis of rotation than the first radial position. The contact structure includes a ring having a groove formed in an outer diameter surface thereof, and the radial support element engages the groove in the contact structure.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a divisional of U.S. patent application Ser. No. 11/708,621, filed Feb. 20, 2007. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to systems for restraining rotatable components, and more particularly to thermally operated systems for restraining rotatable components of turboalternators. 
     Foil bearings are a known type of bearing structure that utilize a thin metal journal lining to support a rotatable shaft and create a hydrodynamic film or air bearing with a working fluid (e.g., xenon gas). For example, certain closed Brayton cycle turboalternators can utilize a turboalternator shaft supported by foil bearings. At operational speeds, the rotating shaft is supported by the fluid pressure of the working fluid and generally does not contact the metal structures of the foil bearings. This means that no wear occurs due to direct physical contact with the rotating shaft during operation, although some contact with metal components of the bearings occurs during startup, shutdown and non-operational periods. 
     However, foil bearings are susceptible to damage, which can reduce or destroy bearing functionality. For instance, with foil bearings used in turboalternators for spacecraft, the turboalternator may not be used during a launch phase of a flight cycle and may only be activated for operation during a later orbital phase of the flight cycle. Because the launch phase will generally subject turboalternator components to stresses, vibration, displacement and other potential sources of damage, it is desired to restrain rotatable components of the turboalternator to prevent damage to the foil bearings during non-operational phases where a hydrodynamic film is not generated and rotatable components can contact the metal structures of the bearings. Active restraint systems, using solenoid actuators or the like, can be used to restrain the rotating components of the turboalternator during the launch phase, but those active systems contain moving parts that present undesirable reliability risks, especially under conditions of extreme ambient temperature variation that occur in aerospace applications. 
     BRIEF SUMMARY OF THE INVENTION 
     A turboalternator system according to the present invention includes a turboalternator having a rotatable member operatively engaged to a bearing set, a radial support element, and a contact structure engaged with the radial support element. The rotatable member defines a first end, a second end and an axis of rotation. The turboalternator system is configured to be thermally adjustable such that in a first thermal condition the contact structure is at a first radial position with respect to the axis of rotation and contacts the rotatable member to provide support, and in a second thermal condition the contact structure is at a second radial position with respect to the axis of rotation that is spaced further from the axis of rotation than the first radial position. The contact structure includes a ring having a groove formed in an outer diameter surface thereof, and the radial support element engages the groove in the contact structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional perspective view of a close Brayton cycle turboalternator having a restraint system according to the present invention. 
         FIG. 2  is a perspective view of the restraint system and turboalternator shaft of  FIG. 1 , shown in isolation. 
         FIG. 3  is a perspective view of a portion of another embodiment of a restraint system according to the present invention. 
         FIG. 4  is a perspective view of a portion of another embodiment of a restraint system according to the present invention. 
         FIG. 5  is a cross-sectional perspective view of the restraint system of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a cross-sectional perspective view of a closed Brayton cycle turboalternator  10  that includes a turbine assembly  12 , a compressor assembly  14 , a rotatable alternator shaft  18 , permanent magnets  20 , a stator assembly  22 , alternator windings  24 , a gas thrust bearing assembly  26 , a cooling fan diffuser  28 , first and second foil bearing assemblies  30 A and  30 B, passive restraint assemblies  32 A and  32 B (collectively, restraint system  32 ), and an alternator housing  34 . The turbine assembly  12  and the compressor assembly  14 , both shown schematically in  FIG. 1  for simplicity, are operably connected to the shaft  18 . In general, the turboalternator  10  operates by converting thermal energy from an external source into rotational energy that turns the shaft  18 . The shaft  18  then rotates the permanent magnets  20  with respect to the stator assembly  22  and the alternator windings  24  in order to generate an electrical current. In this respect, the turboalternator  10  can operate in a conventional manner as will be understood by those of ordinary skill in the art, and therefore it is not necessary to discuss further details of the configuration and operation of the turboalternator  10 . However, it should be noted that the turboalternator  10  of  FIG. 1  is shown by way of example and not limitation, and the present invention is equally applicable to turboalternators having other known configurations. 
     The shaft  18  is operatively supported by the first and second foil bearing assemblies  30 A and  30 B, which are, in turn, supported by bearing carriers  36  and the housing  34 . The foil bearing assemblies  30 A and  30 B can be of a conventional type where, during operation, when the shaft  18  is rotating, the shaft  18  is supported by the fluid pressure of a working fluid (e.g., xenon gas) present between the shaft  18  and metallic structures of the foil bearing assemblies  30 A and  30 B. During operation, the rotating shaft  18  generally does not contact the metal components of foil bearing assemblies  30 A and  30 B. This means that generally no wear occurs due to direct physical contact between the rotating shaft  18  and the metallic structures of the foil bearing assemblies  30 A and  30 B during operation, although some incidental contact may occur. 
     The turboalternator  10  can be installed in a space shuttle or other aerospace vehicle (not shown) that typically will undertake a flight cycle that includes a launch phase, where the turboalternator  10  is not operational, and an orbital phase, where the turboalternator  10  is activated and operated. The restraint system  32  helps to secure the rotatable shaft  18  of the turboalternator  10  when not operational, such as during the launch phase, in order to help reduce the possibility of damage to the foil bearing assemblies  30 A and  30 B due to undesired movement of the shaft  18 . 
       FIG. 2  is a perspective view of the restraint system  32  and the shaft  18 , shown in isolation for clarity. As shown in  FIG. 2 , the shaft  18  defines a first end  18 A and an opposite second end  18 B, and further defines an axis of rotation A. The first restraint assembly  32 A is positioned relative to the first end  18 A of the shaft  18 , and the second restraint assembly  32 B is positioned relative to the second end  18 B of the shaft  18 . The first and second restraint assemblies  32 A and  32 B are substantially identical in the illustrated embodiment, although the assemblies  32 A and  32 B could differ in alternative embodiments. 
     Each of the restraint assemblies  32 A and  32 B includes a ring  38  that is positioned about the shaft  18  (i.e., to encircle the shaft  18 ) and secured to the housing  34  (shown in  FIG. 1 ), three extensions  40 ,  42  and  44  that extend radially inwardly from the ring  38  toward the shaft  18 , and pads  48 ,  50  and  52  (pads  52  are not visible in  FIG. 2 ) that are supported by the extensions  40 ,  42  and  44 , respectively. The extensions  42 ,  44  and  46  are substantially equally spaced from each other and each curve toward the shaft  18  in a spiral-type configuration. The pads  48 ,  50  and  52  are fixed to the radially inner ends of the extensions  40 ,  42  and  44 , respectively, and have curved faces configured to form contact surfaces that can contact the shaft  18 . As shown in  FIG. 2 , the restraint system  32  is engaged such that the pads  48 ,  50  and  52  are in contact with the shaft  18 . Optional circumferential grooves  54 A and  54 B are formed along an outer surface of the shaft  18  relative to each restraint assembly  32 A and  32 B, and the pads  48 ,  50  and  52  extend at least partially into the grooves  54 A and  54 B when engaged. 
     The extensions  40 ,  42  and  44  are bimetallic structures that each comprise two layers  56  and  58  that are bonded or otherwise secured together. The radially outer layer  56  comprises a first material, and the radially inner layer  58  comprises a second material. The second material has a greater coefficient of thermal expansion than the first material, such that changes in ambient temperature cause the extensions  40 ,  42  and  44  to change shape to move the pads  48 ,  50  and  52  relative to the shaft  18 . The first and second materials of the extensions  40 ,  42  and  44  can be bonded together using direct metal deposition, friction welding, or other suitable techniques. The restraint system  32  is configured such that increases in temperature cause the pads  48 ,  50  and  52  to move away from the shaft  18 , while decreases in temperature cause the pads  48 ,  50  and  52  to move toward the shaft  18 . Any materials having differing coefficients of thermal expansion can be used the first and second materials, for example, aluminum and steel. The particular materials used can be selected as a function of the particular thermal operating conditions for a particular application. It should be noted that the rotor  18  typically comprises a material with a low coefficient of thermal expansion, such as a nickel-based superalloy like Inconel®, and therefore is assumed to experience no change in size due as a result of temperature changes. The ring  38  can be made of a material having a coefficient of thermal expansion that is similar or identical to that of a material of the housing  34 . 
     When installed in the turboalternator  10 , the restraint system  32  is configured so that the pads  48 ,  50  and  52  contact the shaft  18  and restrain the shaft  18  when ambient temperatures in the turboalternator  10  are relatively low. Engagement of the pads  48 ,  50  and  52  in the optional grooves  54 A and  54 B provides some restraint in the axial direction, in addition to restraint provided in generally radial directions. As used herein, the term “restraining” means to limit displacement of the shaft  18  relative to the axis of rotation A. A first thermal condition is defined at relatively low temperature conditions when the turboalternator  10  is in a non-operational state and the restraint system  32  is engaged, such as during a launch phase of a flight cycle, and relates to a range of temperatures that are below an operating temperature of the turboalternator  10 . The particular operating temperature of the turboalternator  10  can vary for different applications. 
     When the turboalternator  10  reaches an operational temperature, the restraint system  32  is configured so that the pads  48 ,  50  and  52  move away from the shaft  18 . A second thermal condition is defined at relatively high temperature conditions when the turboalternator  10  is in an operational state and the restraint system  32  disengages, such as during an orbital phase of a flight cycle, and relates to a range of temperatures that are at least as high as a minimum operating temperature of the turboalternator  10 . In the second thermal condition, the radial distance between the pads  48 ,  50  and  52  increases relative to the axis of rotation A of the shaft  18  such that a gap is formed between the pads  48 ,  50  and  52  and the outer surface of the shaft  18 . The gap can vary as desired for particular applications and is typically determined as a function of the configuration of the foil bearings  30 A and  30 B, however a gap of about 0.0254 mm (0.001 inch) or more will generally be sufficient. In the second thermal condition, the shaft  18  is essentially unrestrained by the restraint system  32 . However, where the gap between the pads  48 ,  50  and  52  and the shaft  18  is small, the pads  48 ,  50  and  52  can permit shaft rotation while acting as “bumpers” to limit incidental displacement of the shaft  18  relative to the axis of rotation A and help maintain proper alignment of the shaft  18 . In that situation, the contact surfaces of the pads  48 ,  50  and  52  can optionally be coated with a suitable dry film lubricant in order to reduce friction if and when the shaft  18  contacts the pads  48 ,  50  and  52  momentarily. 
     The temperature of the restraint system  32  is affected by ambient environmental temperatures as well as thermal energy from the external source that powers the turboalternator  10  during operation. More particularly, a coolant medium (e.g., lithium) will generally be warmed to the point of liquification before the turboalternator  10  is activated. As the coolant medium is heated and circulated, for instance, when passed through heat exchangers (not shown), thermal energy will radiate and conduct through the turboalternator  10  and to the restraint system  32 . Generally, a thermal conduction path within the turboalternator is formed through the housing  34  and then to the rings  38  and extensions  40 ,  42  and  44 . 
     An optional heater can be connected to any of the restraint assemblies  32 A and  32 B in order to directly provide thermal energy to the restraint system  32 . An electric heater  60  connected to the ring  38  of the restraint assembly  32 A is shown schematically in  FIG. 2 . The heater  60  can be used to help disengage the restraint system  32  more quickly, or to make disengagement of the restraint system  32  independent from the conduction of thermal energy through the turboalternator  10  from an external source. 
     It is contemplated that the restraint system of the present invention can have alternative embodiments.  FIG. 3  is a perspective view of a portion of another embodiment of a restraint system  132  engaged to a portion of a shaft  18 , shown in isolation. The restraint system  132  includes a restraint ring  138 , three struts  140 ,  142  and  144 , and pads  148 ,  150  and  152  (pad  152  is not visible in  FIG. 3 ). The ring  138  is positioned about the first end  18 A of the shaft  18 . The struts  140 ,  142 ,  144  extend radially inward from the ring  138 , and the pads  148 ,  150  and  152  are supported by the struts  140 ,  142 ,  144 , respectively. The ring  138  comprises a first material, and the struts  140 ,  142 ,  144  comprise a second material. The first material is selected to have a relatively high coefficient of thermal expansion, while the second material is selected to have a relatively low coefficient of thermal expansion. 
     In general, the operation of the restraint system  132  is similar to the restraint system  32  described above in that during a first thermal condition the restraint system  132  is engaged to the shaft  18  and at a higher temperature second thermal condition the restraint system  132  disengages. However, unlike the restraint system  32 , the restraint system  132  operates due to the increase in a radial dimension of the ring  138  as the temperature of the restraint system  132  increases. The struts  140 ,  142 ,  144  undergo little or no change in size as temperature of the system  132  increases, but instead the struts  140 ,  142 ,  144  move the pads  148 ,  150  and  152  relative to the surface of the shaft  18  and the axis of rotation A as the radial dimension of the ring  138  changes. 
     The housing  34  to which the ring  138  is secured can be made of a material with a coefficient of thermal expansion that is similar or identical to the ring  138 , in order to accommodate the changes in radial dimension of the ring  138  while still maintaining secure mechanical support. 
       FIG. 4  is a perspective view of a portion of another embodiment of a restraint system  232  engaged to a portion of a shaft  18 , shown in isolation.  FIG. 5  is a cross-sectional perspective view of the restraint system  232 . The restraint system  232  includes an outer ring  238 , seven springs  240 A- 240 G (springs  240 E and  240 F are not visible in  FIG. 4 ), and an inner ring  248 . The outer ring  238  is positioned to about the first ends  18 A of the shaft  18 , and is spaced from the outer surface of the shaft  18 . The springs  240 A- 240 G extend radially inward from the outer ring  238  in a substantially equally circumferentially spaced spiral-type configuration, and each of the springs  240 A- 240 G acts as a leaf spring. The inner ring  248  is positioned adjacent to the outer surface of the shaft  18  and acts like a circular pad for restraining the shaft  18  like the pads described above. A circumferential groove  270  is formed on a radially outer face of the inner ring  248  between a pair of axially spaced ramp structures  272 A and  272 B. The springs  240 A- 240 G are engaged in the groove  270 , which secures the inner ring  248  relative to the out ring  238 . The ramp structures  272 A and  272 B have a slope that facilitates assembly, by allowing the inner ring  248  to be slid into engagement inside the springs  240 A- 240 G. 
     The inner ring  248  comprises a first material having a relatively high coefficient of thermal expansion, and the outer ring  238  comprises a second material that can have a lower coefficient of thermal expansion. The springs  240 A- 240 G can be formed unitarily with the outer ring  238  and of the same material (i.e., the second material). 
     In general, the operation of the restraint system  232  is similar to the restraint systems  32  and  132  described above in that during a first thermal condition the restraint system  232  is engaged to the shaft  18  and at a higher temperature second thermal condition the restraint system  232  disengages. However, unlike the restraint systems  32  and  132 , the restraint system  232  operates due to the increase in a radial dimension of the inner ring  270  as the temperature of the restraint system  232  increases. As the temperature of the restraint system  232  increases, the inner ring  270  increases in a radial dimension as the first material expands. In the second thermal condition, a gap is formed between the inner diameter of the inner ring  270  and the outer surface of the shaft  18 . Spring force of the springs  240 A- 240 G helps keep the inner ring  270  centered about the axis of rotation A of the shaft  18 . 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and scope of the invention. For instance, the particular configuration of the restraint system according to the present invention can vary as desired for particular applications. Furthermore, the restraint system of the present invention can be utilized with nearly any type of rotatable component. Moreover, optional features described above, such as circumferential grooves in the shaft, dry film lubricants, and heaters, can be utilized with any embodiment of the present invention.

Technology Classification (CPC): 5