Abstract:
An apparatus and method for maintaining separation of two journal foil bearing assemblies, where each journal foil bearing assembly comprises one or more foils with lugs formed thereon for insertion into axial grooves of a bore, the means comprising a pin for insertion into each groove so that an edge of the lug that is inserted into the groove abuts the pin and is prevented from axially drifting along the bore. The pin provides a broad area along the lug edge so that the lug edge does not cut into the pin.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates generally to foil bearings used as journal bearings and spring assemblies associated with the foil bearing, and more particularly to a method and apparatus for maintaining a foil journal bearing and its associated spring assembly (if any) in axial position along a shaft within a bore of housing. 
         [0002]    Fluid film bearings, also known as foil bearings in the prior art, are used in many diverse applications requiring high speed rotating turbo-machinery. A foil bearing generally comprises two relatively movable elements separated by a thin film of fluid lubricant, such as air, refrigerant, and other such fluids. For example, a foil bearing may comprise a stationary element that surrounds a rotating shaft journal, having predetermined radial clearance therebetween filled with air. The foil bearing may or may not be accompanied by an arcuate corrugated spring to assist in maintaining the optimum geometry of the foil bearing around the shaft. 
         [0003]      FIG. 1  shows a cutaway drawing of a typical (one piece) turbo compressor housing  100  with a bore  110  therethrough to receive a rotating shaft (not shown). Midway through the bore  110 , a channel  120  may be found which has a slightly larger inner diameter than that of the bore  110 . As shown in  FIG. 1 , the bore  110  is shown as configured for two foil bearings, with each foil bearing being positioned along a region designated herein as a foil bearing surface  130 , which comprises an inner wall  140  of the bore. At either end of the bore is located a ring retainer groove  150 . 
         [0004]      FIG. 2  shows an exploded view of how the shaft, the foil bearing system  240 , and the housing  100  interrelate, according to the prior art. The foil bearing system  240  is assembled surrounding the shaft  160  and may be comprised of one or more journal foil bearing assemblies  200  with a sleeve  220  therebetween to hold them in a fixed, spaced apart relationship with each other and with respect to the bore  110 . As part of the foil bearing system  240 , retaining rings  230  are used on either end of the foil bearing system  240  to hold the bearing assembly in a fixed position within the bore  110  of the housing  100 . The foil bearing system  240  is assembled by inserting a retaining ring  230  into a ring retainer groove  150  at one end of the bore  110  in the housing  100 . Then a journal foil bearing assembly  200  is inserted against the retaining ring  230 , followed by the sleeve  220  and another journal foil bearing assembly  200 . When the last journal foil bearing assembly  200  has been inserted, the foil bearing assembly  240  may be held in place by inserting a second retaining ring  230  into the remaining ring retainer groove  150  at the opposite end of the bore  110 . The shaft  160  may then be inserted through the foil bearing system  240  and supported thereby. 
         [0005]    Referring now to  FIG. 3 , a cross sectional view of a journal foil bearing assembly  200  is shown. According to the figure, each journal foil bearing assembly  200  comprises one or more springs  202 , which in turn may be fabricated from thin corrugated metal sheets with a retaining lug  210  along the sheet. The springs  202  are interposed between the bore and a foil  203  adjacent the shaft  160  and held from contact by the fluid. The foil  203  may also have a downturned retaining lug  210  formed along its extent. For purposes of this disclosure, there is no functional difference between a lug  210  formed in a spring  202  or a foil  203 . Therefore, when reference is made to a retaining lug  210  hereafter, the reference should be interpreted as being either for a spring  202 , a foil  203 , or both. Furthermore, the retaining lug  210  may be formed in a number of ways, i.e. a downturned edge of the spring/foil or a welded bar running axially along the spring/foil intermediate its radial edges. The manner of forming a retaining lug  210  is not relevant to this disclosure and it should encompass any method and manner of providing a retaining lug  210  for preventing radial movement of the spring  202  or foil  203  about the centerline of the bore  110 . 
         [0006]    Generally, the springs  202  are identical in shape and are fabricated to traverse the circumference of a shaft  160  and occupy the space between the shaft  160  and the inner wall  140  of the bore  110 . It should be noted that different applications may require different numbers of springs  202  and foils  203 , and some applications may dispense altogether with the springs  202 . The configuration shown in  FIG. 3  is typical and used to illustrate the general concept only. 
         [0007]    Significantly, the foil bearing assembly  200  must be restrained from rotating with the shaft. Therefore, each of the springs  202  and the foils  203  may have a retaining lug  210  formed along its outer surface away from the centerline of the shaft  160  in such a way as to align the spring/foil parallel with a centerline of the shaft  160  and bore  110  surrounding the shaft  160 . For this purpose, a number of axial grooves  170  may be machined into the inner wall  140  of the bore  110  such that they are parallel with the centerline. The retaining lug  210  is formed to fit into an axial groove  170  in the inner wall  140  of the bore  110  to prevent the spring  202  or the foil  230  (and thus the journal foil bearing assembly  200 ) from rotating with the shaft  160  and to maintain its position along the foil bearing surface  130 . The axial groove  170  generally runs the extent of the bore  110 . 
         [0008]    A turbo compressor machine may have one or more journal foil bearing assemblies  200  upon which the shaft  160  rotates. Typically, two such assemblies  200  are configured within the bore  110  of the housing  100 . Each journal foil bearing assembly  200  must be held in place along the journals of the shaft  160 . Standard retaining rings  230  are used to constrain axial movement of the journal foil bearing assemblies  200  on the outboard ends of the shaft  160 , where each retaining ring  230  is held in a retaining ring groove  150  (FIG.  1 ). 
         [0009]    However, on the inboard side of each journal foil bearing assembly  200 , installation or use of a retaining ring  230  is difficult due to space limitations, particularly when the housing is fabricated from a single casting. Current practice is to install a single coiled up sleeve  220  fabricated of thin sheet metal in the channel  120  between the two foil bearing surfaces  130  ( FIGS. 1 and 2 ). The sleeve  220  has an axial slit which allows the sleeve  220  to be compressed into a smaller diameter, i.e. a diameter less than that of the bore  110 , so that the sleeve  220  can be inserted into the bore  110  and allowed to “spring back” into its original diameter which coincides with the inner diameter of the channel  120 . 
         [0010]    Referring to  FIG. 4 , the journal foil bearing assembly  200  is shown as it is held in place by the sleeve  220 . According to the figure, the axial groove  170  extends of sufficient depth along the inner wall  140  so that the axial groove  170  opens into the channel  120 . This opening allows a sleeve edge  221  to abut a lug edge  212 , and thereby prevent the journal foil bearing assembly  200  from drifting inwardly along the shaft  160 . The sleeve edge  221  and the lug edge  212  are in approximate 90° relationship to one another and therefore the point of contact is approximately the width of the lug edge  212  and the sleeve edge  221 , which is very small in area. Since this point of contact is so small, the sleeve edge  221  and the lug edge  212  can easily cut into or wear into the other. 
         [0011]    As can be seen, an improved mechanism is needed to maintain axial separation of the foil bearings along the shaft without excessive wear. 
       SUMMARY OF THE INVENTION 
       [0012]    In one aspect of the present invention, a bearing system is provided for supporting a shaft for rotational movement, where the bearing system comprises a first foil bearing assembly for insertion between the shaft and an inner wall of a bore, the first foil bearing assembly comprising one or more foils, each foil curved in an arc around the shaft, each foil having a lug adapted for insertion into an axial groove in the inner wall, the axial groove parallel with a centerline of the bore, wherein the lug constrains the foil from rotational movement about the shaft; and a pin for insertion into the axial groove, the pin having a first end and a second end, the first end abutting an edge of the lug in the groove. 
         [0013]    In a further aspect of the present invention, a turbo compressor is provided, where the turbo compressor comprises a housing; a bore through the housing with the bore having an axial groove extending along an inner wall of the bore, the axial groove being parallel with a centerline of the bore; a shaft received by the bore; an foil bearing assembly positioned between the shaft and the inner wall, the foil bearing assembly comprising a foil curved in an arc around the shaft and a lug adapted for insertion into the axial groove, the lug having a lug end, wherein the lug constrains the foil from rotational movement about the shaft; and a pin positioned in the axial groove, the pin having a first pin end and a second pin end, the first pin end positioned against the lug end, wherein the lug end abuts the first pin end to constrain the foil from axial movement along the bore. 
         [0014]    In another aspect of the present invention, a method for restraining journal foil bearing assemblies from migration along a bore comprises the following steps: positioning the lug of a foil of a first journal foil bearing assembly in an axial groove in an inner wall of the bore and parallel with a centerline of the bore, so that the lug prevents the foil from moving radially about the bore with a shaft inserted therethrough; and preventing the foil from migrating axially along the bore by positioning a pin with a first end and a second end between the foil and an object, the first end abutting the lug of the foil and the second end abutting the object. 
         [0015]    These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  shows a cutaway perspective drawing of a turbo compressor housing and a bore therethrough, according to the prior art; 
           [0017]      FIG. 2  shows an exploded parts drawing, in perspective, of an arrangement of foil bearing assemblies as they are configured along a shaft in the turbo compressor housing bore, according to the prior art; 
           [0018]      FIG. 3  shows a cross sectional view of the bearing assembly of  FIG. 2  having three foils equidistantly positioned about a shaft, according to the prior art; 
           [0019]      FIG. 4  shows a cutaway perspective drawing of a bore in the turbo compressor housing shown in  FIG. 1  in order to illustrate the relationship between the axial grooves, the channel, the sleeve and the lugs, according to the prior art; 
           [0020]      FIG. 5  shows a perspective drawing of two bearing assemblies in which the sleeve has been replaced by a plurality of pins, according to an embodiment of the invention; 
           [0021]      FIG. 6  shows a close up view of an axial groove and the relationship between an end of the pin and the lugs of the bearing assembly, according to an embodiment of the invention; 
           [0022]      FIG. 7  shows an exploded parts drawing, in perspective, of an arrangement of foil bearing assemblies and pins as they are configured along a shaft in the turbo compressor housing bore, according to an embodiment of the invention; and 
           [0023]      FIG. 8  shows a flow diagram of a method for restraining a foil bearing from migration along a shaft, according to an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    The following detailed description is of the best currently contemplated modes of carrying out exemplary embodiments of the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
         [0025]    Various inventive features are described below that can each be used independently of one another or in combination with other features. 
         [0026]    The invention may find application in any machine featuring a rotating shaft which maintains a rotational speed sufficient to justify the use of an foil bearing. In particular, such machines employing a plurality of foil bearings along the same shaft may use the invention to maintain the foil bearings in proper relationship along the shaft. The invention may increase the time between replacement of the foil bearings due to excessive wear along the lugs of the foil bearings. 
         [0027]    Broadly, embodiments of the present invention generally provide a method and a restraint device which replaces the sleeve of the prior art with one or more pins that provide improved wear resistance to the lug edges. The pins may function as a component of a bearing system that may comprise a plurality of foil bearing assemblies and sets of pins correspond in number to the number of lugs associated with springs and foils which comprise each foil bearing assembly. 
         [0028]    Referring now to  FIG. 5 , an embodiment of the present invention may be seen. According to the embodiment, the sleeve  220 , as described above with reference to  FIG. 2  of the prior art, may be replaced by one or more pins  300 . The ends of each pin  300  may abut the lug edges  511 , for as many lugs as may be configured for the springs  520  and foils  530  comprising the foil bearing. As illustrated in the embodiment of the drawing as shown in  FIG. 5 , two journal foil bearing assemblies  500  may be shown, each assembly comprising a single foil  530  with a lug  510  having a lug edge  511 . In addition, each assembly may also comprise three springs  520 , each having a lug  520  having a lug edge  511 . Note that the lug  510  of the foil  530  may be coincident with a lug  510  of one of the springs  520 . Each pin  300  may be inserted into that portion of the axial groove  170  that traverses the channel  120  (as shown in the prior art in  FIGS. 1 ,  3 , and  4 ). While a channel  120  (see  FIGS. 1 and 4 ) may not be necessary according to embodiments of the present invention, such a channel  120  may continue to be fabricated in order to remove metal from the bore  110  and thus reduce weight of the housing  100 . 
         [0029]    The geometry of the axial grooves  170  into which lugs  510  are inserted may be modified to accommodate the pins  300  and enable the pins  300  to be inserted from an end of the bore  110  and slid into place. The pins  300  may fill the axial space between the two journal foil bearing assemblies  500  ( FIG. 5 ). 
         [0030]    Both foil bearing surfaces  130  and the space between them may be combined into a single long bore  110  having a constant diameter, with the axial grooves  170  extending between the ends of the bore  110 . The pins  300  may thereby provide axial separation of the two journal foil bearing assemblies  500  by occupying the axial groove  170  therebetween. The pins  300  may be provided as a set of pins having a number of pins equal in number to the number of foils in the journal foil bearing assembly  500 . However, housing material may be removed to form a channel  120  between the two journal foil bearing assemblies  500  for purposes of saving weight. The channel  120  may be shorter in length than that of the pins  300  so that the pins  300  may still be secured in place at their ends. 
         [0031]    In another embodiment, the journal foil bearing assembly  500  may be positioned away from the end of the bore  110 , so that a retaining ring  530  may be difficult to insert internally to the bore  110 . In such a scenario, pins  300  may be inserted into the axial grooves  170  between the ring retainer groove  150  and the journal foil bearing assembly  500  and then captured in place by installing a retainer ring  530  into the ring retainer groove  150 . 
         [0032]    Referring now to  FIG. 6 , a close up view of an axial groove and the relationship between an end of the pin and the lugs of the foil bearing assembly may be seen. According to  FIG. 6 , an end of pin  300  may be seen as it occupies the recess of axial groove  170 . Note that the geometry of axial groove  170  may be altered as by machining to capture pin  300  and to hold it away from the bore  100 . Lug edge  511  of lug  510  may occupy the same axial groove  170  and rest against the end of pin  300  to be prevented from moving along the bore  110 . 
         [0033]      FIG. 7  shows an exploded view of how the shaft  760 , the bearing system  740 , and the housing  700  interrelate, according to an embodiment of the invention. The bearing system  740  may be assembled surrounding the shaft  760  and may be comprised of one or more journal foil bearing assemblies  780  having a set of pins  300  therebetween to hold them in a fixed, spaced apart relationship. As part of the bearing system  740 , a plurality of retaining rings  785  are placed on either end of the bearing system  740  to hold the bearing system  740  in a fixed position within the housing  700 . The bearing system  740  may be assembled by inserting a retaining ring  785  into a ring retainer groove  750  at one end of the bore  710  in the housing  700 . A journal foil bearing assembly  780  may be inserted against the retaining ring  785 , with the lugs  782  thereof being placed into the axial grooves  770  along the bore  710 , followed by the set of pins  300  and another journal foil bearing assembly  780 . The ends of each pin  300  may abut a lug edge (not shown) of each lug  782  occupying the same axial groove  770  as the pin  300 . When the last journal foil bearing assembly  780  has been inserted, the bearing system  740  may be held in place by inserting a second retaining ring  785  into the remaining ring retainer groove  750  at the opposing end of the bore  710 . The shaft  760  may then be inserted through the bearing system  740  and supported thereby. 
         [0034]    In another embodiment of the invention, a method for restraining journal foil bearing assemblies from migration along a bore may be provided. Referring to the flow diagram  800  shown in  FIG. 8 , the method may be applied for an journal foil bearing assembly having one or more foils, each formed as an arc generally conforming to the curvature of a bore, each foil having a downturned, extending retaining lug that may extend radially away from a centerline of the bore. According to the block labeled  810 , each lug of the journal foil bearing assembly may be positioned in an axial groove extending along an inner wall of the bore and parallel with its centerline. In this manner, the lug may be captured by the axial groove so that it is prevented from moving radially about the bore with a shaft that is inserted therethrough. Each foil and spring, and thus the journal foil bearing assembly, may be held in axial position along the bore by abutting an edge of the lug with a first end of a pin, the second end of which abuts an object, according to the block labeled  820 . 
         [0035]    The opposing end of each pin may abut against another object, so that the foils of the journal foil bearing assembly are maintained in general axial alignment with each other. When two journal foil bearing assemblies are present, each pin may be installed therebetween so that the ends of the pin may abut the corresponding lugs of the assemblies which are inserted therein. Ordinarily the journal foil bearing assemblies may be installed so that one end of each lug abuts a pin in the axial groove and the other end of each lug abuts a retaining ring inserted into a ring retainer groove radially fabricated at an end of the bore. In such a situation, the pins may be regarded as “internal pins”, since they are placed between two journal foil bearing assemblies internally to the bore. 
         [0036]    However, under certain conditions, it may be desirable to space a foil bearing assembly inwardly along the bore away from the normal placement of the ring retainer groove. In such situations, a pin may be inserted into each axial groove so that one end of the pin abuts the retaining ring and the other end of the pin abuts a lug, thus maintaining the journal foil bearing assembly a fixed distance away from and end of the bore. In such a situation, the pins may be regarded as “external pins”, since they are placed immediately adjacent the retaining ring at the end of the bore. 
         [0037]    Under still other conditions, it may be desirable to separately restrain two sets of journal foil bearing assemblies, each set having axial grooves for lugs of that set and only that set. For example, two sets of journal foil bearing assemblies may be provided, with each set containing a plurality of journal foil bearing assemblies each having three foils. Six axial grooves may be provided by the method, with one set of journal foil bearing assemblies assigned three axial grooves and the other set assigned the other three axial grooves. Thus, it may be possible to provide pins for each set of journal foil bearing assemblies to be installed in the grooves assigned to that set of journal foil bearing assemblies, and bypassing the other set. Other permutations may be evident upon inspection. The method does not depend upon the number of foils and springs in each foil bearing assembly or whether or not each foil bearing assembly has the same number of foils and springs. Furthermore, while the number of axial grooves may typically be no greater than the sum of the extending from each of the foil bearing assemblies comprising the foil bearing system for the shaft, the method does not impose a limitation on the manner in which the axial grooves are assigned to different journal foil bearing assemblies, the number of journal foil bearing assemblies having lugs occupying the same axial groove, or that two journal foil bearing assemblies separated by pins be physically adjacent along the shaft. Different geometries and arrangements of this nature may be considered to be within the scope of the invention. The invention does not impose an upper limit on the number of axial grooves that may be provided, and additional unassigned axial grooves may be present for other purposes without restricting the scope of the invention. 
         [0038]    It should be noted that the invention does not impose a restriction on the cross sectional geometry of the pins or on their continuity. For illustration, the pins shown in the drawings have a circular cross section, but other cross sections may be used and still be considered as within the scope of the invention; for example, pins may have a rectangular, square, oval, or oblate spheroid cross section depending upon the application. The invention and method do not depend on the cross sectional geometry of the pins, and each pin associated with a journal foil bearing assembly may have a different cross sectional geometry without departing from the scope of the invention. Furthermore, although each pin has been described as being a single. monolithic object, the pin may be sectioned into a plurality of separate portions abutting each other and all enclosed within a groove, without departing from the scope of the invention. For example, each pin may have its end portions fabricated from a material having a composition that is peculiar to the application and its mid section portion fabricated from a different material for purposes of weight reduction. The portions may be mechanically joined or unconnected within the axial groove, as long as the geometry of the axial groove is such that the axial groove maintains the portions in alignment without allowing any portion to migrate out of the axial groove. 
         [0039]    The disclosure has referred to retaining rings inserted within grooves as being objects against which the foil bearing or the pins abut, in order to maintain required axial spacing of the foil bearing along the shaft. The retaining rings functionally terminates the axial groove at the ends of the bore. However, it should be noted that the use of retainer rings is illustrative and other means may be used to provide a fixed surface against which the foil bearing or the pin abuts. For example, the housing may be divided into two portions and attached together (as by bolts or welding), with a portion of one housing providing the fixed surface for terminating the axial grooves along the bore of the other housing. Present technology makes it difficult to fabricate axial grooves that are “stopped” at the ends, that is, imposed along the inner wall of the bore with the ends of the axial groove ending a short distance from the ends of the bore thus providing a fixed surface against which the end of a pin may abut. Should such technology be developed, then stopped axial grooves may be used to contain pins according to the scope of the invention. 
         [0040]    It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.