Abstract:
A compressor is driven by a drive shaft rotatably supported by a main bearing housing and a lower bearing housing. The main bearing housing positions a main bearing at or near a nodal point of the drive shaft during vibration by the drive shaft. The main bearing housing is designed such that the main bearing pivots during the vibration of the drive shaft in order to maintain surface contact between the bearing and the drive shaft to eliminate edge loading of the bearing.

Description:
FIELD OF THE INVENTION 
     The present invention relates to scroll machines. More particularly, the present invention relates to a scroll compressor which has a main bearing housing with an elastic center which is designed to coincide with the drive shaft&#39;s nodal point corresponding to the first mode of vibration of the drive shaft. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     A class of machines exists in the art generally known as scroll machines which are used for the displacement of various types of fluid. The scroll machines can be configured as an expander, a displacement engine, a pump, a compressor etc. and the features of the present invention are applicable to any one of these machines. For purposes of illustration, however, the disclosed embodiment is in the form of a hermetic refrigerant scroll compressor. 
     Scroll compressors are becoming more and more popular for use as compressors in both refrigeration as well as air conditioning applications due primarily to their capability for extremely efficient operation. Generally, these machines incorporate a pair of intermeshed spiral wraps, one of which is caused to orbit relative to the other so as to define one or more moving chambers which progressively decrease in size as they travel from an outer suction port toward a center discharge port. An electric motor is provided which operates to drive the orbiting scroll member via a suitable drive shaft affixed to the motor rotor. In a hermetic compressor, the bottom of the hermetic shell normally contains an oil sump for lubricating and cooling purposes. 
     The electric motor typically includes a motor stator which is press fit into a shell of the compressor. The drive shaft is typically press fit to the motor rotor and it is rotatably secured by a main bearing housing and a lower bearing housing. Each bearing housing is also secured to the shell of the compressor. During compressor operation, the drive shaft undergoes a nominal static deflection due to the net force on the drive shaft, and as a resultant dynamic load from various excitation sources. The inventors of the present invention have found that a major contribution to the sound levels of the operating compressor in the lower frequency bands is due to the vibration of the drive shaft. 
     The behavior of the drive shaft exhibits a nodal point (zero transverse displacement) in the vicinity of the main bearing of the main bearing housing. The kinetics of the drive shaft with respect to the main bearing of the main bearing housing suggest that the stress in the main bearing will be excessive, primarily because of the localized edge loading from the drive shaft. The localized edge loading is due in part to the rigidity of the main bearing housing which supports the main bearing. This excessive stress being induced in the main bearing due to edge loading can lead to excessive wear of the main bearing and eventually the bearing will wear out prematurely thus reducing the operational life of the compressor. In addition, the dynamic part of this load can be transmitted to the shell of the compressor and causes it to generate noise. 
     The present invention provides the art with a unique main bearing housing which is designed to locate the loaded drive shafts nodal point at the elastic center of the main bearing to eliminate edge loading and its associated problems. The main bearing housing of the present invention is designed to be compliant in the area supporting the main bearing which will locate the nodal point of the drive shaft closer to the elastic center of the main bearing. The compliancy in the mounting of the main bearing by the main bearing housing improves the drive shaft to main bearing contact distribution and further aids in the elimination of edge loading. 
    
    
     Other advantages and objects of the present invention will become apparent to those skilled in the art from the subsequent detailed description, appended claims and drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings which illustrate the best mode presently contemplated for carrying out the present invention: 
     FIG. 1 is a vertical cross section of a hermetic scroll compressor incorporating the unique main bearing housing in accordance with the present invention; 
     FIG. 2 is a vertical cross section of the main bearing housing shown in FIG. 1; 
     FIG. 3 is a top perspective view of the main bearing housing shown in FIG. 2; 
     FIG. 4 is a bottom perspective view of the main bearing housing shown in FIG. 2; 
     FIG. 5 is a graph which illustrates a typical sound spectrum produced by a prior art compressor; and 
     FIG. 6 is a graph illustrating the kinematics of the drive shaft with respect to the main bearing housing in both a typical construction and an ideal construction. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings in which like reference numerals designate like or corresponding parts throughout the several views, there is shown in FIG. 1 a scroll compressor incorporating the unique main bearing housing in accordance with the present invention and which is indicated generally by the reference numeral  10 . Scroll compressor  10  comprises a generally cylindrical hermetic shell  12  having welded at the upper end thereof a cap  14  and at the lower end thereof a base  16  having a plurality of mounting feet (not shown) integrally formed therewith. Cap  14  is provided with a refrigerant discharge fitting  18  which may have the usual discharge valve therein. A transversely extending partition  20  is affixed to shell  12  by being welded about its periphery at the same point that cap  14  is welded to shell  12 . A compressor mounting frame  22  is press fit within shell  12  and is supported by the end of base  16 . Base  16  is slightly smaller in diameter than shell  12  such that base  16  is received within shell  12  and welded about its periphery as shown in FIG.  1 . 
     Major elements of compressor  10  that are affixed to frame  22  include a two-piece main bearing housing assembly  24 , a lower bearing housing  26  and a motor stator  28 . A drive shaft or crankshaft  30  having an eccentric crank pin  32  at the upper end thereof is rotatably journaled in a bearing  34  secured within main bearing housing assembly  24  and a second bearing  36  secured within lower bearing housing  26 . Crankshaft  30  has at the lower end thereof a relatively large diameter concentric bore  38  which communicates with a radially outwardly positioned smaller diameter bore  40  extending upwardly therefrom to the top of crankshaft  30 . The lower portion of the interior of shell  12  defines an oil sump  44  which is filled with lubricating oil to a level slightly above the lower end of a rotor  46 , and bore  38  acts as a pump to pump lubricating fluid up crankshaft  30  and into bore  40  and ultimately to all of the various portions of compressor  10  which require lubrication. 
     Crankshaft  30  is rotatably driven by an electric motor which includes stator  28 , winding  48  passing therethrough and rotor  46  press fitted on crankshaft  30 . An upper counterweight  50  is secured to crankshaft  30  and a lower counterweight  52  is secured to rotor  46 . 
     The upper surface of two-piece main bearing housing assembly  24  is provided with a flat thrust bearing surface  54  on which is disposed an orbiting scroll member  56  having the usual spiral vane or wrap  58  extending upward from an end plate  60 . Projecting downwardly from the lower surface of end plate  60  of orbiting scroll member  56  is a cylindrical hub  62  having a journal bearing  64  therein and in which is rotatively disposed a drive bushing  66  having an inner bore in which crank pin  32  is drivingly disposed. Crank pin  32  has a flat on one surface which drivingly engages a flat surface formed in a portion of the inner bore of drive bushing  66  to provide a radially compliant driving arrangement, such as shown in assignee&#39;s U.S. Letters Pat. No. 4,877,382, the disclosure of which is hereby incorporated herein by reference. An Oldham coupling  68  is also provided positioned between orbiting scroll member  56  and two-piece bearing housing assembly  24 . Oldham coupling  68  is keyed to orbiting scroll member  56  and to a non-orbiting scroll member  70  to prevent rotational movement of orbiting scroll member  56 . 
     Non-orbiting scroll member  70  is also provided with a wrap  72  extending downwardly from an end plate  74  which is positioned in meshing engagement with wrap  58  of orbiting scroll member  56 . Non-orbiting scroll member  70  has a centrally disposed discharge passage  76  which communicates with an upwardly open recess  78  which is in turn is in fluid communication with a discharge muffler chamber  80  defined by cap  14  and partition  20 . An annular recess  82  is also formed in non-orbiting scroll member  70  within which is disposed a floating seal assembly  84 . 
     Recesses  78  and  82  and floating seal assembly  84  cooperate to define axial pressure biasing chambers which receive pressurized fluid being compressed by wraps  58  and  72  so as to exert an axial biasing force on non-orbiting scroll member  70  to thereby urge the tips of respective wraps  58  and  72  into sealing engagement with the opposed end plate surfaces of end plates  74  and  60 , respectively. Floating seal assembly  84  is preferably of the type described in greater detail in assignee&#39;s U.S. Pat. No. 5,156,539, the disclosure of which is hereby incorporated herein by reference. Non-orbiting scroll member  70  is designed to be mounted for limited axial movement to two-piece main bearing housing  24  in a suitable manner such as disclosed in the aforementioned U.S. Pat. No. 4,877,382 or assignee&#39;s U.S. Pat. No. 5,102,316, the disclosure of which is hereby incorporated herein by reference. 
     The present invention is directed to the unique design for main bearing housing assembly  24 . Main bearing housing assembly  24  comprises a main bearing housing  90  and a thrust plate  92 . Thrust plate  92  is secured to main bearing housing  90  using a plurality of bolts (not shown). Thrust plate  92  defines flat thrust bearing surface  54  on which is disposed orbiting scroll member  56  a flat surface  94  on which Oldham coupling  68  is supported. 
     Referring now to FIG. 2, main bearing housing  90  comprises a generally circular section  100  which supports thrust plate  92 . A plurality of legs  102  (four in the embodiment shown) extend radially outward from circular section  100 . In the embodiment illustrated, the outer surface  104  of each leg  102  defines an effective diameter that provides a clearance with shell  12 . Each leg  102  includes an upstanding tower  106  through which extend a mounting hole  108 . Mounting holes  108  are utilized to secure main bearing housing  90  to compressor mounting frame  22  using bolts  110  as shown in FIG.  1 . In another embodiment of the present invention (not shown), the outer surface  104  of each leg  102  defines an effective diameter that is press fit into shell  12 . In this embodiment, mounting hole  108  in each tower  106  is eliminated because main bearing housing  90  is attached to shell  12  and not directly attached to mounting frame  22 . The inner surface  112  of each tower  106  is machined to radially support thrust plate  92 . 
     Main bearing housing  90  further comprises a frusto-conical web  114  which is angled downwardly to support a cylindrical section  116 . Frusto-conical web  114  extends from the lower end of circular section  100  to the lower end of cylindrical section  116 . Cylindrical section  116  defines an inner bore  118  within which bearing  34  is press fitted. The design of main bearing housing  90  with frusto-conical web  114  and cylindrical section  116  provides compliancy of main bearing housing  90  to improve dynamic alignment of main bearing  34  and cylindrical section  116  and thereby improve the reliability of compressor  10  and reduces the transmission of the dynamic load from crankshaft  30  to shell  12 . Main bearing housing  90  with frustoconical web  114  and cylindrical section  116  can be designed to position the loaded drive shaft nodal point at the elastic center of main bearing  34  if desired. 
     Drive shaft  30  is loaded at crank pin  32  which drivingly engages orbiting scroll  62  as well as being loaded by upper counterweight  50  and lower counterwieght  52 . Main bearing  34  and lower bearing  36  provide points for reaction forces to these loads. This combination of forces bends drive shaft  30 . The bent shape of drive shaft  30  corresponds to its instantaneous loading conditions. To describe the bending throughout the rotation of drive shaft  30 , the bending can be seen as an average shape plus the dynamic variation of load with the position of crank pin  32 . Thus, the main bearing journal of drive shaft  30  is not parallel to the axis of compressor  10  by some angle, and the direction of this angle varies with the rotation of drive shaft  30 . It is a significant and separately motivated effort to achieve elastic matching of the primary curvature of the loaded drive shaft  30 . By improving this matching, main bearing  34  and circular section  116  deflect into alignment with the bent main journal of drive shaft  30 . An excessively stiff main bearing housing web  114  prevents main bearing  34  and cylindrical section  116  from deflecting into parallel alignment with the main journal of drive shaft  30  and thus yields top edge loading. An excessively soft main bearing housing web  114  allows main bearing  34  and cylindrical section  116  to deflect more than drive shaft  30  and thus yields bottom loading. Cylindrical section  116  should be designed to be stiff enough to act as a solid body to support main bearing  34 . An excessively thin cylindrical section  116  allows the top portion of cylindrical section  116  to deflect away from the journal load and yields center loading with insufficient distribution of the load to the upper section of main bearing  34 . 
     It is a second significant achievement to match the dynamic variation in the curvature of drive shaft  30  due to vibration so that edge loading does not break down the oil film to yield metal-to-metal contact and thereby prevent wear of main bearing  34 . It is a third significant achievement to position main bearing  34  at the node of drive shaft  30  which minimized the transmission of the vibration of drive shaft  30  to main bearing housing  90  and the surrounding environment. 
     The envelope of a sound spectrum produced by a prior art compressor has a unique and easily recognizable shape. The sound spectrum exhibits two “humps” whose location in the spectrum shifts slightly depending upon the compressor size. The inventors of the present invention have associated the groups of frequency bands in the sound spectrum with specific components of the compressor as shown in FIG.  5 . The “hump” on the right side or upper half of the frequencies of the sound spectrum has been attributed to the top cap of the compressor which typically has its natural frequencies in that part of the frequency range. The excitation source is the discharge gas impinging upon the top cap. The “hump” on the left side or lower half of the frequencies is caused by a variety of circumstances and the inventors of the present invention have determined that a major contribution to the sound levels in these lower frequency bands is due to the vibration of the drive shaft. 
     Referring now to FIG. 6, the vibration behavior of the drive shaft in a prior art compressor exhibits a nodal point (zero transverse displacement) in the vicinity of a main bearing housing  130  as shown in the broken line of FIG. 6 in the absence of main bearing  34 . Ideally, the nodal point is located at the elastic center of main bearing  34  as shown in the solid line  132  of FIG.  6 . When the nodal point is not located at the elastic center of main bearing  34  (the broken line  134  of FIG.  6 ), the stress on the bearing will be excessive due to the localized edge loading from the drive shaft. Frusto-conical web  114  is designed to produce a vibration behavior as shown by the solid line in FIG.  6 . The design of web  114  and its interface with both circular section  100  and cylindrical section  116  provides the necessary compliancy to the system which elastically matches the shaft and the bearing which significantly reduces the edge loading. The edge loading is reduced due to the elastic matching of the shaft and the bearing allowing the bearing to flex when the shaft vibrates at its natural frequency. 
     While the above detailed description describes the preferred embodiment of the present invention, it should be understood that the present invention is susceptible to modification, variation and alteration without deviating from the scope and fair meaning of the subjoined claims.