Patent Publication Number: US-7901194-B2

Title: Shaft coupling for scroll compressor

Description:
BACKGROUND 
     The present invention is directed to fluid compressors suitable for use with vapor-compression cycles and, more particularly, to shaft couplings for orbiting scroll compressors. 
     Orbiting scroll compressors utilize opposing scrolls to compress a working fluid between two disks along a spirally wound compression path. A stationary scroll includes a first disk having a first spiral wound flange facing an orbiting scroll. The orbiting scroll includes a second disk having a second spiral wound flange that intermeshes with the first spiral wound flange. The first and second spiral wound flanges are disposed between the first and second disks to form a spiral shaped flow path. The second scroll is offset from the first scroll such that the second flange contacts the first flange at intervals of approximately every half-winding of the flow path. As such, the orbiting scroll orbits around the center point of the stationary scroll such that fluid trapped between contact points of the flanges is compressed as it works its way from between the outer windings to between the inner windings as the radius of the windings and the volume of the flow path decrease. 
     In order to provide the orbiting action of the orbiting scroll, the second disk is connected to a drive shaft through a bearing shaft. The bearing shaft is connected to the drive shaft through a bearing socket having a central axis offset from a central axis of the drive shaft. As the drive shaft rotates about its central axis, the central axis of the bearing socket rotates about, or orbits, the central axis of the drive shaft. As the second flange of the orbiting scroll engages the first flange of the stationary scroll to compress the fluid along the flow path, rotation of the orbiting scroll about the central axis of the bearing shaft is prevented and the bearing socket rotates around the bearing shaft. Thus, the bearing socket and bearing shaft are subject to three-dimensional torque from the mechanical coupling of the drive shaft and the scroll, as well as from the pressure of the compressed fluid flowing through the flanges. 
     Due the different performance requirements of the scroll and the bearing shaft, it has been typical practice to fabricate the scroll and the bearing shaft from different materials. For example, scrolls are typically comprised of a relatively soft, lubricious material suitable for allowing contact between the flanges. Conversely, bearing shafts are typically comprised of relatively hard, wear-resistant materials suitable for engagement with bearings. It is generally cost-prohibitive to fabricate the scroll from bearing material and performance-prohibitive to fabricate the bearing shaft from scroll material. It therefore becomes necessary to join these components through a coupling that permits each component to function properly and that can withstand the forces transmitted during the compression process. Previous coupling designs have relied on the strength of a single, small diameter threaded fastener that extends through the bearing shaft and the orbiting scroll. The small diameter bolts of these designs are susceptible to breaking and produce stress concentrations within the orbiting scroll, thus limiting the operating speed and power of the compressor. As such, there is a need for a shaft coupling for use in an orbiting scroll compressor that provides suitable material performance and torque transmitting characteristics. 
     SUMMARY 
     The present invention is directed to a coupling mechanism for a scroll compressor. The coupling mechanism comprises an orbiting scroll disk, a retention bolt, a bearing shaft and a retention nut. The orbiting scroll disk includes a first face configured to engage a stationary scroll disk to compress a working fluid, and a second face having a hub. The retention bolt is inserted into the hub. The bearing shaft is fit onto the retention bolt and includes a bearing surface for engaging a drive bushing of a drive shaft. The retention nut is threaded onto the retention bolt to retain connection of the bearing shaft with the orbiting scroll disk. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a diagrammatic, cross sectional view of a scroll compressor in which a shaft coupling of the present invention is used to connect a drive shaft to an orbiting scroll. 
         FIG. 2  shows a shaft coupling for connecting a bearing shaft with a scroll hub in the scroll compressor of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a cross sectional view of scroll compressor  10  having shaft coupling  12  of the present invention. Scroll compressor  10  includes hermetic shell  14 , electric motor  16 , drive shaft  18 , bearing shaft  20 , orbiting scroll  22  and stationary scroll  24 . Shell  14  comprises a casing in which components of compressor  10  are hermetically sealed so that a fluid, such as a refrigerant, can be directed to scrolls  22  and  24  to be compressed in a contaminant-free environment. Scroll compressor  10  is configured to receive low pressure fluid F LP  at inlet  26  of shell  14 , compress the fluid utilizing stationary scroll  24  and orbiting scroll  22 , which is driven by motor  16 , and discharge high pressure fluid F HP  at outlet  28  of shell  14 . In the embodiment shown, shell  14  comprises three segments  14 A,  14 B and  14 C connected at bolted flanges  30  to facilitate assembly and maintenance of compressor  10 . Additionally, shell segment  14 A includes cover  15  to provide access to motor  16  and shaft  18 . Bearing shaft  20  joins coupler  32  of drive shaft  18  and hub  34  of orbiting scroll  22  so that drive shaft  18  is linked with orbiting scroll  22  within shell  14 . Shaft coupling  12  of the present invention connects bearing shaft  20  with hub  34  to reduce stress concentrations within hub  34  and bearing shaft  20 . 
     Electric motor  16  comprises an electromagnetic motor having stator  36  and rotor  37 . In the embodiment shown, stator  36  includes wire windings  38  mounted to shell segment  14 B, and rotor  37  includes a plurality of permanent magnets  39  mounted on drive shaft  18 . Stator  36  and rotor  38  operate as is known in the art as a conventional electric drive motor to produce rotation of shaft  18  about central axis CA. In other embodiments, however, other types of drive motors may be used. Drive shaft  18  rotates on central axis CA within bearings  40 A and bearings  40 B, which are supported within shell  14  by struts  42 A and  42 B, respectively. Bearings  40 A comprise ball bearings and are configured to ride directly on shaft  18  near shell segment  14 A. Bearings  40 B comprise roller bearings and are configured to support shaft  18  at coupler  32  near shell segment  14 C. Shaft  18  extends from strut  42 A at shell segment  14 A, through electric motor  16  within shell segment  14 B, to strut  42 B at shell segment  14 C. As such when, motor  16  is activated, such as when electric current is supplied to windings  38  of stator  36 , rotor  37  is electro-magnetically driven to rotate about central axis CA, causing drive shaft  18  to also rotate about central axis CA. 
     Coupler  32  comprises cylindrical head  43 , which is positioned at an end of shaft  18  and includes bore  44 . Head  43  is centered on shaft  18  such that head  43  rotates generally uniformly about central axis CA when drive shaft  18  rotates. Bore  44 , however, is positioned within head  43  such that bearing axis BA of bore  44  is offset a distance x from central axis CA. As such, the center of bore  44  and bearing axis BA orbit central axis CA when shaft  18  rotates. Bearing  48  is disposed within bore  44  and is configured to receive bearing shaft  20  such that the center of bearing shaft  20  also orbits central axis CA. In the embodiment shown, bearing  48  comprises a roller bearing, but in other embodiments other bearings or bushings may be used. Utilizing coupling  12  of the present invention, bearing shaft  20  joins hub  34  of orbiting scroll  22  with coupler  32  and drive shaft  18 . Thus, coupler  32  operates as a cam to provide the orbiting motion that drives orbiting scroll  22  against stationary scroll  24 . 
     Orbiting scroll  22  includes hub  34 , orbiting disk  50 , and orbiting scroll flange  52 . Similarly, stationary scroll  24  includes stationary disk  54 , stationary scroll flange  56  and reed valve  58 . Stationary scroll  24  is mounted to shell segment  14 C within compressor  10  through any suitable means as is known in the art such that stationary scroll  24  remains generally immobile during operation of compressor  10 . Orbiting scroll  22  is supported by shaft  18  through the connection of bearing shaft  20  with hub  34  and coupler  32 . Orbiting scroll  22  is positioned such that orbiting scroll flange  52  is inter-disposed with stationary scroll flange  56  to form a flow path having intermittent contact between flange  52  and flange  56 . Flanges  52  and  56  comprise wraps that form a spiral compression path that winds from the outer diameters of disks  50  and  54  toward central axis CA. Stationary disk  54  is mounted to shell segment  14 C such that an innermost portion of scroll flange  56  is generally aligned with central axis CA. Orbiting disk  50  is mounted on bearing shaft  20  such an innermost portion of scroll flange  54  is generally aligned with bearing axis BA. The offset distance x provides the gyrating action of orbiting disk  54  when shaft  18  rotates such that the center of scroll flange  52  orbits around central axis CA within scroll flange  56 . Bearings  48  rotatably connect bearing shaft  20  with coupler  32  to prevent binding of orbiting flange  52  within stationary flange  56 . Thus, bore  44  and bearings  48  rotate around bearing shaft  20  while the center of bearing shaft  20  orbits central axis CA on bearing axis BA. As such, orbiting scroll  22  and stationary scroll  24  operate conventionally to compress a fluid along the flow path. 
     Low pressure fluid F LP  enters compressor  10  at inlet  28  at shell segment  14 A. Low pressure fluid F LP  flows into shell segment  14 B and surrounds electric motor  16 . Stator  36  and rotor  38  include passages or channels that permit low pressure fluid F LP  to pass through motor  16 . Low pressure fluid F LP  flows through channels  60  and into shell segment  14 C such that the fluid is disposed radially about scrolls  22  and  24  in suction chamber  61 . Low pressure fluid F LP  is sucked into the spiral flow path of flanges  52  and  56  by the orbiting action of scroll  22 . From within the compression path, a small amount of compressed fluid is bled through small bores (not shown) in disk  50  to provide lubrication to bearings  40 A,  40 B and  48 . Compressed fluid is pushed into interior channel  62  extending through bearing shaft  20  and then into bore  44  of coupler  32 . From the outer periphery of bore  44 , the compressed fluid winds through and lubricates bearings  40 B and bearings  48  before being discharged into shell segment  14 B. Additionally, from a center portion of bore  44 , the compressed fluid exits coupler  32  and enters channel  63  within shaft  18  to lubricate bearings  40 A, before discharging into shell segment  14 B. The fluid returned to shell segment  14 B from bearings  40 A,  40 B and  48  is recycled into the compression cycle where it is again delivered to suction chamber  61  and the compression flow path formed by flanges  52  and  56 . 
     Orbiting scroll flange  52  engages stationary scroll flange  52  to compress and push low pressure fluid F LP  toward central axis CA, whereby the fluid is discharged into pressure chamber  64  through reed valve  58  as high pressure fluid F HP . Reed valve  58  discharges high pressure fluid F HP  from scrolls  22  and  24  in pulsed bursts and prevents backflow of fluid into scrolls  22  and  24 . Pressure chamber  64  also provides a damping chamber for attenuating the pulses of compressed high pressure fluid F HP  released by reed valve  58 . High pressure fluid F HP  is pushed out of compressor  10  at outlet  28  in shell segment  14 C whereby the compressed high pressure fluid F HP  is available for use, such as in a vapor-compression system. In one embodiment of the invention, compressor  10  provides compressed refrigerant for use in an aircraft refrigeration and air conditioning system. Compressor  10  also includes other components, such as resolver  65  and economizer inlet  66 , to facilitate operation of compressor  10  and the vapor-compression system. 
     Shaft  20  connects coupler  32  of shaft  18  to hub  34  such that orbiting scroll  22  is provided with the orbiting motion necessary to compress fluid with stationary scroll  24 . As such, bearing shaft  20  is subjected to various three-dimensional loading due to the mechanical torque transmission from shaft  18  and the fluid compression process from scroll  22 . For example, bearing shaft  20  is subject to bending forces from both bearings  48  and hub  34 . Likewise, scroll flange  52  contacts scroll flange  56  to cause stress on disk  50  and hub  34 . These various forces require different material properties for bearing shaft  20  and scroll  22 . It is desirable for bearing shaft  20  to be comprised of a somewhat hard material suitable for engaging bearing  48 . It is, however, desirable for scroll  22  to be comprised of a somewhat soft material to foster engagement of flanges  52  and  56 . Coupling  12  of the present invention provides a mechanism that permits bearing shaft  20  and orbiting scroll  22  to be fabricated from materials that permit optimal performance of each component. Additionally, coupling  12  provides a mechanism that joins shaft  20  to hub  34  to prevent the formation of stress concentrations within orbiting scroll  22  and shaft  20 . 
       FIG. 2  shows coupling  12  for connecting bearing shaft  20  with orbiting scroll  22 . Coupling  12  includes bearing shaft  20 , hub  34 , disk  50 , connector  67  and retainer  68 . Hub  34  includes axial flange portion  70 , socket  72  and notch  74 . Connector  67  includes lubrication bore  62 , head  76 , shank  78  and axial recess  82 . Shaft  20  includes bearing surface  84 , radial flange  86 , axial flange  88 , assembly bore  90  and retainer bore  92 . As described above, the center of orbiting scroll  22  is configured to orbit around central axis CA of drive shaft  18  ( FIG. 1 ), while bearing  48  and bore  44  of coupler  32  ( FIG. 1 ) rotate about bearing shaft  20 . As such, shaft  20  is comprised of a somewhat hard material to transmit torque from shaft  18  to scroll  22  and to provide a durable bearing surface for bearing  48 . Scroll  22  is, however, comprised of a somewhat pliable or supple material for engaging stationary scroll  24 . Coupling  12  mechanically engages the disparate materials of shaft  20  and scroll  22 , while distributing stress throughout the coupling. 
     Scroll  22  is configured to be mounted within compressor  10  such that orbiting scroll flange  52  interlocks with stationary scroll flange  56  to form a flow path for compressing a fluid. A first surface of disk  50  provides a portion of the flow path and seals the edges of flanges  52  and  56 . A second surface of disk  50  includes socket  72 , which joins disk  50  with bearing shaft  20 . Axial flange  70  of socket  72  extends axially from disk  50  such that flange  70  is concentrically disposed about bearing axis BA. Similarly, socket  72  extends into disk  50  such that socket  72  is concentrically disposed about bearing axis BA. In one embodiment of the invention, socket  72  extends into disk  50  an approximate equal length as flange  70  extends out of disk  50 . Flange  70  and socket  72  include threads on their interior facing surfaces to receive head  76  of connector  67 . 
     Connector  67  comprises a T-shaped fastener or connector having head  76  and shank  78 . Head  76  includes threads that mate with threads within flange  70  and socket  72  such that connector  67  is rigidly connected to hub  34 . Head  76  is threaded into flange  70  and socket  72  such that the width of head  76  spans the transition region between flange  70  and socket  72 . Shank  78  of connector  67  comprises a transition shaft that extends axially from head  76  along bearing axis BA. Shank  78  includes lubrication bore  62  to permit a lubrication fluid to flow through coupling  12 . For example, lubrication bore  62  fluidly connects the second surface of scroll disk  50  with bore  44  of coupler  32  ( FIG. 1 ). Axial recess  82  extends into head  76  concentrically about shank  78  and is configured to receive axial flange  88  of bearing shaft  20 . Assembly bore  90  of bearing shaft  20  is positioned around shank  78  such that shank  78  extends into retainer bore  92 . Bearing shaft  20  engages with connector  67  and hub  34  such that axial flange  88  enters axial recess  82  of connector  67  and radial flange  86  contacts axial flange  70  of hub  34 . In one embodiment of the invention, axial flange  88  is press-fit or snap-fit into axial recess  82  to couple bearing shaft  20  with connector  67 . Shank  78  includes threads such that retainer  68  can be fastened to connector  67 . Retainer  68  comprises a nut having threads configured to mate with threads of shank  78  such that retainer  68  can be tightened onto shank  78  to push bearing shaft  20  into tight contact with hub  34  and scroll  22 . Retainer  68  includes notches  94  such that a tool or machine can be employed to apply torque to retainer  68 , particularly once retainer  68  is positioned within retainer bore  92 . For example, in one embodiment of the invention, a push pole device is used to preload shank  78 . A push pole or similar device applies pre-tension to shank  78  before positioning retainer  68  onto shank  78 . When the pre-tension is relieved on shank  78 , retainer  68  is pulled straight into retainer bore  92  to engage bearing shaft  20  and secure retainer  68  with a more pure axial tension, avoiding production of twisting or three-dimensional torsional stresses in shaft  20  and shank  78 , and avoiding forces that can loosen retainer  68 . Because of the threaded engagement between head  76 , flange  70  and socket  72 , stress from retainer  68  is dispersed over a wide surface area of hub  34 , rather than being concentrated on scroll  22 . Thus, shank  78  assists in transitioning the tension applied by retainer  68  into hub  34 . In one embodiment, shank  78  is preloaded with ten thousand pounds of tension. 
     Connector  67  of coupling  12  brings bearing shaft  20  into a rigid and solid engagement with scroll  22  to distribute loading and to minimize stress concentrations within hub  34 . The threaded engagements between connector  67 , hub  34  and retainer  68  inhibit separation between shaft  20  and scroll  22 , thus preventing damage to axial flange  70  and radial flange  86 . The diameters of head  76  and hub  34  are sized to be nearly as large as the diameter of shaft  20  such that stresses generated at the interface are spread over a large surface area. The diameter of shank  78  is, however, smaller such that the structural integrity of bearing shaft  20  is not compromised. Head  76  is seated within hub  34  such that head  76  contacts both flange  70  and socket  72  to avoid the creation of stress concentrations within scroll  22 . For example, socket  72  is recessed into disk  50  to prevent flange  72  from bearing all of the bending stresses applied to shaft  20  from coupler  32 . Socket  72  also includes notch  74 , which extends concentrically around bearing axis BA where socket  72  and disk  50  converge, to provide stress relief within scroll  22 . Socket  72  distributes loading into disk  50 , which has a greater thickness and mass than flange  70 . Flange  70 , however, enables the depth of socket  72  to be greater than is the thickness of disk  50  such that additional surface area is provided for engagement with head  76  of connector  67 . The depth of socket  72 , including flange  70 , is greater than the thickness of head  76 . Head  76  is not completely threaded into socket  72  such that head  76  does not contact disk  50  where it is thinned to form socket  72 . Head  76  is, however, threaded far enough into socket  72  such that head  76  is completely recessed into socket  72 . Head  76  is inserted into socket  72  such that axial flange  88  of shaft  20  is able to engage axial recess  82 , and radial flange  86  of shaft  20  is able to engage flange  70 , enhancing the stability of coupling  12 . Radial flange  86  contacts axial flange  70  to provide radial stability to bearing shaft  20  and prevent bending stresses. Axial flange  88  inhibits axial movement of bearing shaft  20 . 
     In one embodiment of the invention, bearing shaft  20  is comprised of hardened steel, such as a tool steel, to provide a smooth and durable surface upon which bearings  48  can rotate. Such steels are, however, expensive, making fabrication of scroll  22  infeasible. Furthermore, machining such steels also requires expensive manufacturing processes that further increase the cost of producing scroll  22  from tool steel. Additionally, it is desirable that scroll  22  be comprised of a relatively softer, more lubricious material. Thus, in one embodiment of the invention, scroll  22  is comprised of a cast material, such as cast iron. Cast iron and other materials of similar hardness provide a measure of self-lubrication in that they are able to yield or deform to absorb small amounts of contact with stationary scroll  24 , such as binding arising from imperfections in the oscillation of orbiting scroll  22 . Scroll  22  can also be produced to include graphite to further facilitate lubricity. Connector  67  can be comprised of any suitable material for providing a threaded engagement with hard and soft materials, such as a 400 series steel. 
     The shaft coupling of the present invention achieves a sturdy connection between a bearing shaft and an orbiting scroll. The shaft coupling includes a transition connector that distributes stress concentrations within a hub of the orbiting scroll. The transition connector pulls the bearing shaft into tight engagement with the orbiting scroll. The transition connector includes a large diameter head that distributes loading within the hub over a large surface area. The head engages both a flange portion and a socket portion of the hub to prevent stress concentrations from forming within the orbiting scroll. The transition connector can also be pre-tensioned to reduce torsional stresses in the bearing shaft. Furthermore, the transition connector permits the bearing shaft and the orbiting scroll to be produced from materials suitable for optimizing performance of each component. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.