Abstract:
A compressor rotor assembly including an impeller including an impeller stem, the stem including a first coupling end having a first face and at least one arcuate coupling tab along the first face; the impeller stem further comprising a bore that extends inwardly from the first face, the bore having an interior wall that is tapered. The rotor assembly further comprising a pinion shaft having a second coupling end with a second face and at least one arcuate coupling slot along the second face; and a hub extending outwardly from the second face, the hub including a tapered outer wall; the first and second coupling means and the hub and bore are adapted to be mated when the impeller and pinion shaft are assembled to prevent relative displacement of the stem and shaft.

Description:
This application is a continuation-in-part of U.S. patent application Ser. No. 09/413,698 filed Oct. 6, 1999 now U.S. Pat. No. 6,254,349 and claims the benefit of provisional application No. 60/142,256 filed Jul. 2, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a device and method for detachably connecting an impeller member to a pinion shaft member in a high speed fluid compressor, and more particularly the invention relates to a connection device and method where one of the members includes at least one tab that is inserted into a corresponding at least one slot provided on the other member. 
     A high speed fluid compressor such as a centrifugal compressor includes a rotor assembly that is comprised of an impeller that is coupled to a pinion shaft which includes a pinion gear that meshes with a drive gear to drive the impeller at high rotational velocities of up to 76,000 rpm, for example. The suitable attachment between the impeller and pinion must be able transmit torque from the pinion gear to the impeller, maintain zero relative motion of the impeller relative to the pinion, permit easy assembly and disassembly of the rotor assembly, and consistently relocate the pinion and impeller at their original relative positions when the components are reassembled. Accurate maintenance of the relative positions of the impeller and rotor is critical to ensure that the rotor assembly retains its dynamic balance. 
     The impeller and pinion shaft are conventionally coupled by a polygon attachment method. The principal advantages of the polygon attachment method are its ease of assembly/disassembly and self centering characteristic. The polygon must consistently lock up the impeller and pinion shaft at the same position to maintain the needed level of rotor balance. Any relative movement between the pinion shaft and impeller leads to unacceptable levels of vibration during compressor operation. To ensure the requisite consistency is obtained, the mating parts must be machined to very exacting tolerances. 
     FIG. 1 illustrates a prior art rotor assembly generally comprised of pinion shaft  12  coupled to an impeller  14  by a polygon attachment method. The pinion shaft  12  includes pinion gear  16  which is engageable with a power transmission assembly (not shown) which drives the pinion about a pinion axis  18  at a predetermined rotational velocity during operation of the centrifugal compressor. The pinion shaft  12  includes a drive end  20  which has formed therein a polygonally dimensioned bore  22 . The polygonally dimensioned bore  22  has an interior bore surface which defines a generally triangular cross section composed of circular arcs. 
     The impeller  14  incorporates a backward-leaning type blade geometry  24 , and the impeller includes a polygonally dimensioned stem portion  26  which is defined by an exterior stem surface  28 . The stem portion  26  includes a first end  26 a and a second end  26   b . The polygonally dimensioned stem portion  26  is suitably matingly dimensioned to be received by the polygonally dimensioned bore  22 . The stem portion  26  is typically dimensioned to have a cross section which deviates from a circular pattern and which has a shape that is convex on all sides and essentially elliptical, triangular or quadratic as illustrated in FIG.  2 . After coupling the pinion shaft and impeller, the pinion shaft is rotated and the lobes along the stem  26  are locked against adjacent portions of bore  22 . 
     The polygon attachment method has a number of shortcomings. The polygon attachment method is useful because it is repeatable and maintains permanent location by its shape. However, if the mating parts are not parallel and the shapes of the lobes are not accurately calculated and precisely machined, as the rotor assembly comes up to speed stresses in the components may alter the shapes of the lobes and as a result loosen the connection between the pinion shaft and impeller. Also, the compressor could experience surge or vibration that occurs during operation and as a result the surge or vibration could displace the impeller to a new location and out of balance. The polygon is expensive and difficult to manufacture. The mating polygon surfaces are difficult to measure for quality and precision. The continuous rubbing and surface contact on highly stressed polygonally shaped parts causes galling and fretting of the parts and the galling and fretting could cause the impeller and pinion shaft to be fused together. 
     The foregoing illustrates limitations known to exist in present devices and methods for assembling impellers and pinion shafts. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter. 
     SUMMARY OF THE INVENTION 
     In one aspect of the present invention, this is accomplished by providing a rotor assembly that includes an impeller including an impeller stem, the stem including a first coupling end having a first face and first coupling means along the first face; and a pinion shaft having a second coupling end with a second face and second coupling means along the second face, the first and second coupling means adapted to be mated when the impeller and pinion shaft are assembled to prevent relative displacement of the impeller and pinion shaft. 
     The first coupling means is comprised of at least one arcuate tab, and the second coupling means is comprised of at least one arcuate slot adapted to receive the at least one arcuate tab when the impeller stem and pinion shaft are mated. Each tab includes an inner arcuate surface, and substantially planar terminating surfaces joining the inner and outer arcuate surfaces; the arcuate tabs having different arclengths and widths. If one tab is included, the tab is simply inserted into the mating slot, and if more than one tab is provided, the tabs are different with different arclengths so that they can only be inserted into their mating slot and in this way the required relative orientation between the stem and pinion shaft is maintained. 
     In addition to the tab/slot coupling structure the pinion shaft includes a hub that extends outwardly from the second face and is adapted to be mated with a bore formed in the impeller stem. The wall of the bore and hub are tapered so that an interference fit is created when the hub is inserted in the bore. 
     In summary, the present invention is comprised of an attachment device and method comprised of a set of tabs/slots and tapered cylindrical hub. The tab/slot feature is used to transmit power between the mated parts and the tab/slot feature limits assembly of the component parts to a single orientation ensuring that the pinion shaft and impeller will be assembled at the same relative position when the parts are disconnected and then reassembled. The tapered cylindrical hub achieves an interference fit between the mating parts, and thus ensures that the two mating parts do not move relatively in the radial dimension. This ensures retention of dynamic balance of the assembly. Also, the interference fit that is achieved, provides additional power transmission capability. This design provides means to achieve the needed joint stiffness, balance retention, and power transmission capabilities while it can more easily be manufactured than the conventional polygon and other attachment methods. 
     The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures. 
    
    
     DESCRIPTION OF THE DRAWING FIGURES 
     FIG. 1 is an exploded, side elevational view of an impeller and a pinion shaft of a prior art rotor assembly for a centrifugal compressor. 
     FIG. 2 is an end view of a polygonally dimensioned stem portion of the prior art impeller illustrated in FIG.  1 . 
     FIG. 3 is a longitudinal sectional view of the impeller and pinion shaft of the rotor assembly of our present invention. 
     FIG. 4 is a lateral sectional view taken along line  4 — 4  of FIG.  3 . 
     FIG. 5 is a perspective view of the coupling end of the impeller shaft of FIG.  3 . 
     FIG. 6 is a perspective view of the coupling end of the pinion shaft of FIG.  3 . 
     FIG. 7 is a longitudinal sectional view of the impeller and pinion shaft of the rotor assembly of an alternate embodiment. 
     FIG. 8 is a lateral sectional view taken along line  8 — 8  of FIG.  7 . 
     FIG. 9 is a perspective view of the coupling end of the impeller shaft of FIG.  7 . 
     FIG. 10 is a perspective view of the coupling end of the pinion shaft of FIG.  7 . 
     FIG. 11 is a longitudinal sectional view of the impeller and pinion shaft of the rotor assembly of another alternate embodiment. 
     FIG. 12 is a lateral sectional view taken along line  11 — 11  of FIG.  11 . 
     FIG. 13 is a perspective view of the coupling end of the impeller shaft of FIG.  11 . 
     FIG. 14 is a perspective view of the coupling end of the pinion shaft of FIG.  11 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning now to the drawings wherein like parts are referred to by the same number throughout the several views, FIGS. 3-6 illustrate the rotor assembly coupling of the present invention. 
     Specifically, FIG. 3 shows the rotor assembly  40  that includes impeller  14  that is made integral with impeller stem  42 , and pinion shaft  44  that includes pinion (not shown) like pinion  16 . The pinion shaft and impeller shaft are detachably joined by assembly coupling  46 . 
     As will be described hereinbelow, the assembly coupling of the present invention ensures that the mating impeller stem and pinion shaft do not move relatively in the radial dimension during compressor operation. The assembly coupling  46  provides means to achieve the needed joint stiffness, balance retention, and power transmission capabilities and it can more easily be manufactured than the conventional polygon and other attachment methods. 
     Turning to FIGS. 4 and 6, the unitary pinion shaft  44  includes a coupling end  61 , a free end  63 , and axis  62 . The coupling end includes a lateral face  64 . A coupling hub  66  extends axially away from face  64  and has a tapered exterior surface that tapers inwardly as the hub extends away from the pinion shaft lateral face  64 . A threaded bore  67  adapted to receive a bolt or another conventional fastener extends along axis  62  through the hub  66  and a portion of the pinion shaft  44 . Opposed arcuate slots  68  and  70  are provided in lateral face  64 . Each slot includes inner and outer arcuate surfaces that are joined by substantially planar terminating surfaces. However, the arcuate slots are not the same and slot  70  has a greater arclength and width than slot  68 . As shown in FIGS. 4 and 6, the slots are separated by approximately 180 degrees. 
     Turning now to FIGS. 4 and 5, the unitary impeller stem portion  42  includes a coupling end  48 , free end  50 , and longitudinal axis  45 . The coupling end  48  terminates in lateral face  49  and free end  50  terminates in lateral face  51 . A substantially cylindrical bore  52  extends inwardly from coupling end face  49  to position within the stem, and the bore  52  includes a wall that is tapered inwardly as it extends inwardly away from the lateral face  49 . See FIG.  4 . The bore terminates at lateral end face  53 , and the end face and inwardly tapered side wall define a cavity  55 . A countersunk bore  54  extends between bore  52  and lateral face  51 . 
     First and second tabs  56  and  58  are provided along lateral face  51 . The tabs are used to accurately and consistently relatively orient and locate the coupled impeller stem and pinion shaft. The tabs extend outwardly from lateral face  49  and are substantially perpendicular to the face and are offset by about 180 degrees. Each tab is substantially arcuate with inner and outer arcuate surfaces joined by substantially planar terminating surfaces. As shown in FIG. 4, first tab  56  includes inner and outer arcuate surfaces  56   a  and  56   b  respectively which are joined by terminating surfaces  56   c  and  56   d , and second tab  58  includes inner and outer arcuate surfaces  58   a  and  58   b  respectively which are joined by terminating end surfaces  58   c  and  58   d . As shown in FIG. 4, the tabs are not the same and have different arc lengths and widths. Tab  58  is adapted to be fitted into slot  70  and tab  56  is adapted to be fitted into slot  68 . In this way, when the rotor assembly is disassembled, it can be assembled so that the impeller and pinion shaft are coupled in the same relative position before they were disassembled. 
     Although two slots and tabs are illustrated and described, it should be understood that any suitable number of mating slots and tabs may be used to obtain and maintain the desired relative positioning and orientation between the pinion shaft and impeller stem. Although in the description the tabs are provided on the stem lateral face  49 , and the slots are provided on the pinion shaft lateral face  64 , it should also be understood that the tabs could be provided on the pinion shaft face  64  and the slots could be provided on lateral coupling face  49 . 
     Assembly and disassembly of the rotor assembly  40  will now be described. When it is necessary to assemble rotor assembly  40 , axes  45  and  62  are aligned and hub  66  is slid into bore  52 . The hub and bore are dimensioned so that as the hub is inserted into the bore an clamping load is produced as a result of the interference fit between the tapered bore and hub surfaces. It has been determined by the coinventors that the resultant clamping load is sufficient to prevent relative movement of the impeller and pinion shaft. 
     As the hub is slid into the bore, tabs  70  and  68  are aligned with their respective slots  58  and  56 , so that the tabs are located in the respective slots when the hub is located in the bore  52 . The tabs ensure the desired relative location of the stem and pinion shaft after the completion of maintenance. After seating an o-ring seal  90  in the large diameter portion of countersunk bore  54 , bolt  92  is passed through bore  54  and bore  67  and is tightened until the ends of the tabs are in contact with the back of the slots. See FIG.  3 . 
     When it is necessary to service the rotor assembly, the bolt  92  is removed and the impeller is displaced axially from its location along the pinion shaft. 
     An alternate embodiment of an assembly coupling  146  is illustrated in FIGS. 7-10. FIG. 7 shows the assembly coupling  146  that detachably joins an impeller stem  142  with a pinion shaft  144 . Similar to the previously described embodiment, the assembly coupling  146  of this alternate embodiment transmits torque and prevents the mating impeller stem  142  and pinion shaft  144  from moving relative to one another in the radial dimension during operation. 
     As shown in FIG. 9, the impeller stem  142  has an outer stem surface  143  around the exterior of the impeller stem  142 , and a first coupling end  148  having a first coupling face  149 . The first coupling face  149  is illustrated as a lateral face at the first coupling end  148  and may be transverse to an impeller axis  145 . A tab  156  projects axially outward from the first coupling face  149 , and terminates at a tab surface  158 . The tab  156  extends across the first coupling face  149  intersecting with the outer stem surface  143 . Two driving surfaces  159  extend along the sides of the tab  156  between the first coupling face  149  and the tab surface  158 . A hub  166  extends axially away from the first coupling face  149 , and has a tapered exterior surface that tapers radially inward as the hub  166  extends away from the first coupling face  149 . 
     FIG. 10 illustrates the pinion shaft  144  having an outer shaft surface  147  around the exterior of the pinion shaft  144 , and a second coupling end  161  having a second coupling face  164 . The second coupling face  164  is illustrated as a lateral face at the second coupling end  161 , and may be transverse to a pinion axis  162 . A slot  168  is formed in the second coupling face  164 , and extends axially inward from the second coupling face  164  terminating at a slot surface  171 . The slot  168  extends across the second coupling face  164  intersecting with the outer shaft surface  147 . Two side walls  170  extend between the second coupling face  164  and the slot surface  171 . A cylindrical bore  152  extends axially inward from the second coupling face  164  to a position within the pinion shaft  144 , and the bore  152  includes a wall that is tapered radially inward as it extends away from the second coupling face  164 . 
     As shown in FIGS. 7-10, the hub  166  is sized to mate with the bore  152  when the impeller stem  142  and pinion shaft  144  are assembled. The mating hub  166  and bore  152  align the impeller stem  142  and pinion shaft  144 , and prevent the impeller stem  142  and pinion shaft  144  from moving relative to one another in the radial direction. The hub  166  and bore  152  arrangement of this embodiment is similar to the previously described embodiment, but in this alternate embodiment the hub  166  extends from the first coupling face  149  on the impeller stem  142 , and the bore  152  extends into the second coupling face  164  on the pinion shaft  144 . This arrangement is reversed from the previous embodiment, shown in FIGS. 5 and 6, which illustrate the hub  66  on the pinion shaft  44  and the bore  52  in the impeller stem  42 . Either arrangement is possible, and the hub  166  and the bore  152  may be disposed at either the first coupling face  149  or the second coupling face  164  as long as both the hub  166  and the bore  152  are present. 
     As illustrated in FIGS. 8-10, the tab  156  has a tab width dimension  180 , and the slot  168  has a slot width dimension  184 . The tab width dimension  180  is the distance between the driving surfaces  159 , and the slot width dimension  184  is the distance between the side walls  170 . In the illustrated arrangement, the tab width dimension  180  is greater than the diameter of the hub  166  at the intersection of the hub  166  and the tab surface  158 . The slot width dimension  184  is greater than the diameter of the bore  152  at the intersection of the bore  152  and the slot surface  171 . 
     As shown in FIGS. 7 and 8, when the impeller stem  142  and pinion shaft  144  are assembled together, the tab  156  and the slot  168  mate with one another to transmit torque between the pinion shaft  144  and impeller stem  142 . The tab  156  fits within the slot  168 , and the side walls  170  are aligned with the driving surfaces  159 . As the pinion shaft  144  rotates about the pinion axis  162 , the side walls  170  contact the driving surfaces  159  and rotate the impeller stem  142  about the impeller axis  145 . 
     As explained above, the tab  156  and slot  168  are arranged to properly align when the impeller stem  142  and pinion shaft  144  are assembled. In FIGS. 9 and 10, the tab  156  and slot  168  may be centered about the impeller axis  145  and pinion axis  162  respectively, or the tab  156  and slot  168  may be offset from each respective axis. When the tab  156  and slot  168  are centered, the driving surfaces  159  are both substantially equidistant from the impeller axis  145 , and the side walls  170  are both substantially equidistant from the pinion axis  162 . With the centered arrangement, the impeller stem  142  and pinion shaft  144  may have two possible mating positions, with each mating position being a 180 degree rotation from the other mating position. 
     When the tab  156  and slot  168  are offset, the distance from the impeller axis  145  to each individual driving surface  159  is different, and the distance from the pinion axis  162  to each side wall  170  is different. Even though the tab  156  and slot  168  are offset, they are equally offset so that the tab  156  and slot  168  still align with one another. With the offset arrangement, the impeller stem  142  and pinion shaft  144  only have one mating position, and will always align at substantially the same orientation to one another when being reassembled. 
     In the illustrated arrangement, the driving surfaces  159  are substantially planar, and are substantially parallel to each other. Also, the side walls  170  are illustrated as substantially planar, and are substantially parallel to each other. Alternatively, the shape of the tab  156  and slot  168  could be altered as long as the corresponding shapes are similar and the tab  156  and slot  168  still mate with one another. For example, the tab  156  could be tapered across the first coupling face  149 , and the slot  168  could be similarly tapered across the second coupling face  164 . The tapered arrangement provides another arrangement in which the impeller stem  142  and pinion shaft  144  would only have one mating position, and would always align at the same orientation to one another when being reassembled. 
     In the previously described embodiment, the tab  156  is disposed on the first coupling face  149 , and the slot  168  is disposed on the second coupling face  164 . Alternatively, the slot  168  could be formed in the first coupling face  149 , and the tab  156  could project outward from the second coupling face  164 . The tab  156  and slot  168  design could be reversed and the assembly coupling  146  would still transmit torque between the pinion shaft  144  and impeller stem  142 . 
     Another alternate embodiment of an assembly coupling  246  is illustrated in FIGS. 11-14. This alternate embodiment uses a hub  266  and a bore  252  arrangement similar to the previous embodiments to align an impeller stem  242  and a pinion shaft  244  radially, but a different interface is used to transmit torque between the impeller stem  242  and pinion shaft  244 . As shown in FIG. 13, a raised elliptical surface  256  projects axially outward from a first coupling face  249 . A driving surface  259  extends along the side of the elliptical surface  256  between the elliptical surface  256  and the first coupling surface  249 . The elliptical surface  256  is substantially parallel to the first coupling face  249 , and is disposed near the intersection of the first coupling face  249  and the hub  266 . 
     As shown in FIGS. 12 and 13, the elliptical surface  256  has a maximum surface dimension  280  and a minimum surface dimension  282 . The maximum surface dimension  280  represents the distance across the elliptical surface  256  at its widest point, and the minimum surface dimension  282  represents the distance across the elliptical surface  256  at its narrowest point. The maximum surface dimension  280  is shown as smaller than the diameter of the first coupling surface  249 . The minimum surface dimension  282  is shown as larger than the diameter of the hub  266  at the intersection of the hub  266  and the elliptical surface  256 . 
     As shown in FIG. 14, an elliptical bore  268  is formed in a second coupling face  264 , and extends axially inward from the second coupling face  164  terminating at a shoulder  271 . A side wall  270  runs around the perimeter of the elliptical bore  268 , and extends from the second coupling face  264  to the shoulder  271 . The shoulder  271  intersects with the tapered wall of the cylindrical bore  252 . 
     As shown in FIGS. 12 and 14, the elliptical bore  268  has a maximum bore dimension  284  and a minimum bore dimension  286 . The maximum bore dimension  284  represents the distance across the elliptical bore  268  at its longest point, and the minimum bore dimension  286  represents the distance across the elliptical bore  268  at its shortest point. The maximum bore dimension  284  is smaller than the diameter of the second coupling surface  264 . The minimum bore dimension  286  is larger than the diameter of the tapered cylindrical bore  252  at the intersection of the bore  252  and the shoulder  271 . 
     As shown in FIGS. 11 and 12, when the impeller stem  242  and pinion shaft  244  are assembled together, the elliptical surface  256  and the elliptical bore  268  mate with one another to transmit torque between the pinion shaft  244  and impeller stem  242 . The elliptical surface  256  fits within the elliptical bore  268 , and the side wall  270  is aligned with the driving surface  259 . As the pinion shaft  244  rotates about a pinion axis  262 , the side wall  270  contacts the driving surface  259  and rotates the impeller stem  242  about a impeller axis  245 . 
     The elliptical surface  256 , as illustrated in FIGS. 12-14, is shown as symmetrical about both the maximum surface dimension  280  and the minimum surface dimension  282 , and centered on the impeller axis  245 . Similarly, the elliptical bore  268  is shown as symmetrical about the maximum bore dimension  284  and the minimum bore dimension  286 , and centered about the pinion axis  262 . With this symmetrical arrangement of the mating elliptical surface  256  and elliptical bore  268 , the impeller stem  242  and pinion shaft  244  may have two possible mating positions, and each mating position being a 180 degree rotation from the other mating position. 
     Alternatively, the elliptical surface  256  and elliptical bore  268  may be non-symmetrical as long as they are still mating. With the non-symmetrical arrangement, the impeller stem  242  and pinion shaft  244  only have one mating position, and will always align at the substantially same orientation to one another. 
     The elements of this alternate embodiment could also be reversed similar to the alternative arrangements of the previously described embodiments. The elliptical surface  256  or the hub  266  could project outward from the second coupling face  264 , and the elliptical bore or the cylindrical bore  252  could extend inward from the first coupling face  249 . The elliptical surface  256  and elliptical bore  252  design could be reversed and the assembly coupling  246  would still transmit torque between the pinion shaft  244  and impeller stem  242 . 
     While we have illustrated and described preferred embodiments of the invention, it is understood that this is capable of modification, and we therefore do not wish to be limited to the precise details set forth, but desire to avail ourselves of such changes and alterations as fall within the purview of the following claims.