Abstract:
In a co-rotational scroll apparatus having two interleaving scroll wraps secured to end plates rotating about parallel, non-aligned axes to produce a relative orbital motion, a mass secured to at least one of the scrolls for enhancing the nutational stability of the scroll member. Preferably, the nutation reducing means includes a mass affixed to an end plate of the scroll member to induce a dynamic imbalance during rotation of the scroll member, creating a balancing or moderating moment sufficient to compensate for the moment induced by the varying pressure of the fluids in the various compression chambers during the rotation of the scroll member.

Description:
TECHNICAL FIELD 
     This invention generally pertains to scroll apparatus and specifically to co-rotating scroll-type fluid apparatus having means for enhancing the stability of one or more of the rotating scroll members. 
     BACKGROUND ART 
     Scroll apparatus for fluid compression or expansion are typically comprised of two upstanding interfitting involute spirodal wraps which are generated about respective axes. Each respective involute wrap is mounted upon an end plate and has a tip disposed in contact or near-contact with the end plate of the other respective scroll wrap. Each scroll wrap further has flank surfaces which adjoin in moving line contact, or near contact, the flank surfaces of the other respective scroll wrap to form a plurality of moving chambers. Depending upon the relative orbital motion of the scroll wraps, the chambers move from the radially exterior end of the scroll wraps to the radially interior ends of the scroll wraps for fluid compression, or from the radially interior end of the respective scroll wraps for fluid expansion. The scroll wraps, to accomplish the formation of the chambers, are put in relative orbital motion by a drive mechanism which constrains the scrolls to relative non-rotational motion. The general principles of scroll wrap generation and operation are discussed in numerous patents, such as U.S. Pat. No. 801,182. 
     Numerous attempts have been made to develop co-rotational scroll apparatus. Such apparatus provides for concurrent rotary motion of both scroll wraps on parallel, offset axis to generate the requisite orbital motion between the respective scroll wrap elements. However, most commercially successful scroll apparatus to date have been of the fixed scroll-orbiting scroll type due to various difficulties in achieving success with co-rotating scroll apparatus. 
     Typically, a number of rotary bearings are required in a co-rotational scroll apparatus, which decreases the reliability and efficiency of the machine. Furthermore, the typical co-rotating scroll apparatus have required a thrust bearing acting upon each of the scroll end plates to prevent axial scroll separation, thus substantially increasing the power requirements of the machine as well as substantially reducing the reliability of the machine. 
     An additional problem which must be dealt with in scroll apparatus, whether used for compression or decompression of fluid, are the forces which result from the fluid trapped in the chambers formed in the scroll wraps. These forces include an axial separation force component resulting from the fluid pressure upon the scroll element end plates and a radial separation force resulting from the fluid pressure upon the scroll wraps themselves. Furthermore, the separation forces due to the fluids compressed within the scroll elements vary cyclicly as the scroll elements rotate. This cyclic variation is a function of two factors. The first is the instantaneous location of each of the compression chambers formed by the scroll wraps during each revolution. The chamber location is a function of the angular and radial disposition of the center of the chamber with respect to the center of the scroll apparatus at a given crankangle. The second is the actual pressure of the compressed fluid, which varys according to the instantaneous location of the compression chamber in which the fluid is contained, decreasing from the radially inner ends of the respective scroll wraps to the radially outer ends thereof. Both these factors combine to produce a moment, the product of the instantaneous center of the compression chamber location and the instantaneous fluid pressure forces at that location. The resulting tipping moment upon the scroll member is the net effect of the moments developed by each compression chamber. The tipping moment acts perpendicularly to the axis of rotation of the scroll member, and therefore seeks to cause the tipping of the scroll element. Since the magnitude of the tipping moment is more pronounced at various crankangle positions during the rotation of the scroll element, actual tipping may occur at some crankangle positions, while it may be prevented at other positions by other forces sufficiently exerted on the scroll members. Actual tipping is observable as a rocking or nutation of the scroll member during rotation. 
     Typically, this is dealt with by the provision of an axial force acting to compress the end plates of the scroll elements together, in opposition to the separating fluid forces and by the provision of relatively larger bearings. These compressive axial forces are typically induced either mechanically by such means as thrust bearings or springs, or by fluid pressure imposed upon the opposite side of the scroll end plate. 
     Prior scroll apparatus attempt to counter the nutation effect by simply increasing the axial force loading upon the scroll end plate until the tipping moments are overcome, by providing a large number of bearings for supporting the scroll member shafts to prevent the shaft misalignment which occurs during tipping, and by decreasing the manufacturing tolerances of the components. All of these solutions increase the size and number of components of the scroll apparatus as well as the initial and operating costs, and also decrease the expected operating life of the scroll apparatus. 
     These solutions also undesirably affect the performance of the scroll apparatus as well. Because the axial force provided remains constant at any given operating condition, the axial force loading remains relatively high even when the separation effects of the tipping moment are low, which is typically the case during most of the scroll rotary cycle. Hence, there are unnecessarily high forces acting upon the scroll wrap tips at many crankangle positions in the scroll cycle, with resulting unnecessary friction and wear as well as excessive power consumption and loss of overall efficiency. 
     Furthermore, even when the axial force loading is relatively high, tipping of the scroll member can occur at some crankangle positions during rotation of the scroll apparatus. When nutation of the scroll element does occur, the scroll wrap tips can momentarily separate from the opposing scroll end plate. This permits fluid to pass from higher pressure compression chambers to lower pressure chambers, requiring recompression of the fluid and again reducing the overall efficiency of the scroll apparatus. 
     Therefore it is an object of the present invention to provide a scroll apparatus as will provide the highest possible efficiency while utilizing the least amount of power and therefore having the lowest power and least costly drive means. 
     It is a further object of the present invention to provide a method of reducing and compensating in a scroll apparatus at least in part for the net moment upon a rotating scroll member. 
     It is still a further object of the present invention to provide such a co-rotating scroll apparatus which is of simple construction and high operating reliability. 
     It is yet a further object of the present invention to provide a co-rotating scroll apparatus which is relatively compliant and not susceptible to damage in operation. 
     Finally, it is an object of the present invention to provide such a scroll apparatus as is suitable for and is relatively inexpensive in mass production. 
     SUMMARY OF THE INVENTION 
     The subject invention is a method and means for enhancing the rotational stability of at least one of the scroll members or elements in a co-rotational scroll apparatus having two concurrently rotating scroll members, each scroll member including an end plate and a scroll wrap thereon having at least an involute portion for interleaving engagement with the scroll wrap of the other scroll member and rotating on an axis parallel to the axis of the other scroll member. 
     Specifically, the subject invention includes a mass disposed on, or alternatively, a mass integral with the scroll end plate of at least one of the scroll members. This mass is disposed near the periphery or outer edge of the scroll end plate. The mass generates a moment which adds to the net effect of the moments generated by fluid forces within the scroll wraps, which is referred to as a tipping moment since tipping of the scroll member can result from the effect of this moment upon the scroll member. The mass is disposed so that the moment acting upon the scroll member as a result of the mass reduces or moderates the moment generated by other forces acting upon the scroll member during the rotation of the scroll member. This enhances the nutational stability of the scroll member during rotation, or in other words, reduces the rocking of the scroll member during rotation. 
     According to the method of the subject invention, the magnitude of the instantaneous moment resulting from fluid forces acting upon the scroll member, or tipping moment, is determined for each angular point or position throughout the rotation of the scroll member. From this, the maximum tipping moment acting upon the scroll member and the range of crankangle positions through which the maximum tipping moment acts can be found. The amount of mass, the radius or distance by which the mass is removed from the axis of rotation of the scroll member, and angular disposition of the mass necessary to induce a sufficiently moderating moment to moderate or reduce the maximum determined tipping moment is then also determined. The appropriate mass is then applied to the scroll member at the radius and angular disposition thus determined to reduce the nutation of the scroll member. 
     An exemplary co-rotational scroll apparatus which may suitably employ the subject invention is also presented. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 discloses a cross-sectional view of a co-rotational scroll apparatus embodying the subject invention. 
     FIG. 2 discloses in schematic representation a refrigeration system in which the subject invention could be suitably employed. 
     FIG. 3 shows a cross-sectional view of the scroll apparatus of FIG. 1 taken along section lines 3--3. 
     FIG. 3A is an enlarged view of the central portion of FIG. 3 which more clearly illustrates the location and offset of the axis of rotation of the drive and idler scroll members as well as the line of zero crank angle and angles phi 1  and phi 2  which are defined with respect thereto. 
     FIG. 4 shows the effect of the tipping moment upon a representative co-rotational scroll apparatus. 
     FIG. 5 is a diagram representative of the combined tipping moment and moderating moment, and of the axial scroll tip contact force acting upon one scroll member during the rotation of the scroll member in a co-rotational scroll apparatus. 
     FIG. 6 is a diagram representative of the tipping moment as combined with various moderating moments, acting upon one of the scroll members during the rotation of the scroll members. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A scroll type fluid apparatus generally shown in FIG. 1 as a scroll compressor assembly is referred to as reference numeral 20. As the preferred embodiment of the subject invention is a hermetic scroll compressor assembly, the scroll apparatus 20 is interchangeably referred to as a scroll compressor 20 or as a compressor assembly 20. It will be readily apparent that the features of the subject invention will lend themselves equally readily to use in a scroll apparatus acting as a fluid expander, a fluid pump, or to scroll apparatus which are not of the hermetic type. 
     In the preferred embodiment, the compressor assembly 20 includes a hermetic shell 22 having an upper portion 24, a lower portion 26, a central exterior shell 27 extending between the upper portion 24 and lower portion 26, and an intermediate, central frame portion 28 affixed within the central exterior shell 27. The exterior shell 27 is a generally cylindrical body, while the central frame portion 28 is defined by a generally cylindrical or annular exterior portion 30 and a central portion 32 disposed across one end thereof. The annular exterior portion 30 of the central frame portion 28 is sized to sealingly fit within the exterior shell 27 so that it may be mated thereto by a press fit, by welding, or by other suitable means. 
     Integral with the central frame portion 28 is a generally cylindrical upper bearing housing 34, which is substantially coaxial with the axis of the annular exterior portion 30. A drive shaft aperture 36 extends axially through the center of the upper bearing housing 34, and an upper main bearing 38 is disposed radially within the drive shaft aperture 36. Preferably, the upper main bearing 38 is made, for example, of sintered bronze or similar material, but may also alternatively be a roller or ball-type bearing, for accepting a rotating load therein. 
     A motor 40 is disposed within the upper portion 24 and central shell portion 28 of the hermetic shell 22. The motor 40 is preferably a single-phase or three-phase electric motor comprised of a stator 42 which is circumferentially disposed about a rotor 44, with an annular space formed therebetween for permitting free rotation of the rotor 44 within the stator 42 as well as the flow of lubricant or refrigerant fluid. 
     It will be readily apparent to those skilled in the art that alternative types of motors 40 and means of mounting motor 40 would be equally suitable for application in the subject invention. For example the stator 42 could be secured within the central shell portion 27 by a press fit therebetween. Alternatively, a plurality of long bolts or cap screws (not shown) may be provided through appropriate apertures in the stator plates into threaded apertures in the central frame portion 28 for securing the motor 40 within the hermetic shell 22. 
     The scroll arrangement includes a first or drive scroll member 76 and a second or idler scroll member 78, each having an upstanding involute scroll wrap for interfitting engagement with the other respective scroll wraps. The first scroll member 76 includes an upstanding first involute scroll wrap 80 which is integral with a generally planar drive scroll end plate 82. The drive scroll end plate 82 includes a central drive shaft 84 extending oppositely the upstanding involute scroll wrap 80. A discharge gallery 86 is defined by bore extending centrally through the axis of the drive shaft 84. The discharge gallery 86 is in flow communication with a discharge aperture 88 defined by a generally central bore through the drive scroll end plate 82. The drive shaft 84 further includes a first, relatively large diameter portion 90 extending axially through the upper main bearing 38 for a free rotational fit therein, and a second relatively smaller diameter portion 92 which extends axially through the rotor 44 and is affixed thereto. The rotor 44 may be affixed to the rotor portion 92 of the drive shaft 84 by such means as a press fit therebetween or a power transmitting key in juxtaposed keyways. 
     The second or idler scroll member 78 includes a second, idler scroll wrap 100 which is disposed in interfitting contact with the driven scroll wrap 80. The idler scroll wrap 100 is an upstanding involute extending from an idler end plate 102. An idler stub shaft 104 extends from the idler end plate 102 oppositely the idler scroll wrap 100. 
     The designation of the drive scroll member 76 as the first scroll member and the idler scroll member 78 as the second scroll member must be understood as arbitrary, made for the purposes of ease of description and therefore not as a limitation. It would be equally accurate to designate the idler scroll member 78 as the first scroll member and the drive scroll member 76 as the second scroll member. 
     An annular bearing 110, which may be a sleeve bearing made of sintered bronze material, or may be of the roller or ball-type, is disposed within an annular wall defining an idler bearing housing 112 which is integral with the lower hermetic shell portion 26 as a support means for rotationally supporting the second or idler scroll member 78. 
     The first scroll end plate 82 also includes two extension members 120 extending from the first scroll end plate 82 parallel the drive scroll wrap 80. The extension members 120 are disposed at radially opposed positions near the outer edge of the first scroll end plate 82 and are of greater length than the height of the involute scroll wraps 80 and 100, respectively, plus the thickness of the second scroll end plate 102. The extension members 120 are affixed to an annular first scroll member compression plate 130. The compression plate 130 is generally cup shaped, having an annular generally planar circumferential portion 132 about the radial outward end thereof, to which the extension members 120 are affixed by such means as threaded fastener, welding or press fit. A depressed planar central portion 136 is parallel to and downwardly spaced a distance from the outer end portion 132 of the compression plate 130. This central portion 136 includes a second, slightly more downwardly spaced area describing an annular retaining shoulder 138 and a biasing surface 140. A central aperture 142 is described by a bore through the axial center of the depressed portion 136. The central aperture 142 is of substantially greater diameter than the lower bearing housing 112 so that there is sufficient clearance between the compression plate 130 and the lower bearing housing 112 to permit the compression plate 130 to rotate freely about the lower bearing housing 112. 
     A compression and drive spring 150 is disposed between the biasing surface 140 and the second scroll end plate 102. The compression spring 150 serves as a biasing means to force the respective scroll end plates 82 and 102 toward each other by exerting a force upon the second scroll end plate 102 and an opposite force upon the first scroll end plate 82 through the compression plate 130 and extension members 120. In the preferred embodiment, the spring 150 is retained within an annular channel 152 formed in the second scroll end plate 102. This permits the spring 150 also to act as a torque transmitting element. In this embodiment, the extension members, the compression plate 130 and the spring 150 together comprise a drive means for causing concurrent rotation of the first scroll member 76 and second scroll member 78. 
     Alternative drive means may include an Oldham-type ring driveably connecting the extension members 120 and drive keys on the idler scroll end plate 82. Since the form of drive means are not particularly relevant to the subject invention, no further detailed discussion thereof is deemed necessary herein. 
     In FIG. 2, the scroll compressor assembly 20 is shown connected at the discharge aperture 50 and the suction aperture 52 to a fluid system such as generally is used in refrigeration or air conditioning systems. Those skilled in the art will appreciate that this is but one fluid system in which the scroll compressor assembly 20 could suitably be utilized, and that application of the scroll compressor assembly 20 in refrigeration and air conditioning systems is to be taken as exemplary rather than as limiting. 
     The refrigeration system, shown generally in schematic representation in FIG. 2 in connection with the scroll compressor assembly 20, includes a discharge line 54 connected between the shell discharge aperture 50 and a condenser 60 for expelling heat from the refrigeration system and in the process typically condensing the refrigerant from vapor form to liquid form. A line 62 connects the condenser 60 to an expansion device 64. The expansion device 64 may be a thermally actuated or electrically actuated valve operated by a suitable controller (not shown), a capillary tube assembly, or other suitable means of expanding the refrigerant in the system. Another line 66 connects the expansion device 64 to an evaporator 68 for transferring expanded refrigerant from the expansion device 64 to the evaporator 68 for the acceptance of heat and typically the evaporation of the liquid refrigerant to a vapor form. Finally, a refrigeration system suction line 70 transfers the evaporated refrigerant from the evaporator 68 to the compressor assembly 20, wherein the refrigerant is compressed and returned to the refrigeration system. 
     It is believed that the general principles of refrigeration systems capable of using suitably a scroll compressor apparatus 20 are well understood in the art, and that further detailed explanations of the devices and mechanisms suitable for constructing such a refrigeration system need not be discussed in detail herein. It is believed that it will also be apparent to those skilled in the art that such refrigeration or air conditioning systems may include multiple units of the compressor assembly 20 in parallel or series type connection, as well as multiple condensers 60, evaporators 68, or other components and enhancements such as subcoolers and cooling fans and so forth as are believed known in the art. 
     FIGS. 3 and 3A present cross-sectional views of FIG. 1 which more clearly disclose the subject invention. A dimension 0 defines the offset distance between the axis D and the axis I. A line phi 0  is defined through the axis D of the drive scroll member 76 and axis I of the idler scroll member 88. Since these axes are fixed, the line phi 0  is also fixed with reference to the scroll apparatus 20 and may in turn be used as a reference line from which the angular disposition of the scroll apparatus components may be referenced. The line phi 0  also represents the point of zero crankangle and the point at which the outer ends of the respective scroll wraps 80 and 100 first make contact with the other respective scroll wrap to close the first or outer chamber. 
     In FIG. 3, an unbalancing or moment reducing mass 160 is applied to the drive scroll member 76, while a second moment producing mass 162 is applied to the idler scroll member 78. As shown, the preferred embodiment of the subject invention employs a mass 160 and 162 applied by such mechanical means as welding or adhesive to the respective scroll member end plate 82 and 102. The masses 160 and 162 comprise means for enhancing the nutational stability of the scroll member to which they are applied, as will be explained below. 
     The moment producing mass 160 has a center of gravity cg 1  which is disposed at a radius r 1  from the center of rotation (axis D) of the first scroll member 76 to which it is applied. The mass 160 is angularly disposed at an angle phil from the line phi 0 . The second moment producing mass 162 has a center of gravity cg 2  disposed at a radius r 2  from the center of rotation (axis I) of the idler scroll member 78. The second mass 162 is applied to the end plate 102 at an angular disposition defined by angle phi 2  from the line phi 0  described above. 
     In the preferred embodiment, the shape of the masses 160 and 162 includes curved surfaces so as to minimize any potential frictional resistance between the masses 160 and 162 and the fluid in which the scroll members 76 and 78 are rotating. It will be appreciated that the shape of the masses 160 and 162 may be varied, and that the masses 160 and 162 may even be formed to act as impeller vanes and thereby assist the inflow of fluid to the scroll wraps 80 and 100 when the scroll apparatus 20 is operated as a compressor. Furthermore, it will be appreciated that the radius r and angle phi for the masses 160 and 162 as shown are purely representative, and not to be taken as limiting. It is likely in many cases that phil and phi 2  will be equal or substantially equal and that in many cases it may be desirable to provide only a mass 160 or a mass 162 on only one of the scroll members. It must also be understood that the mass m 1  of mass 160 may or may not be substantially equal to the mass m 2  of the second mass 162 in a scroll apparatus which includes both the first mass 160 and the second mass 162. The amount of the mass m 1  and m 2  of the first mass 160 and second mass 162, the radius r 1  and r 2  by which the masses are removed from the respective axis of rotation, and the radial disposition phi 1  and phi 2  of the masses must be determined according to each particular case according to the teaching below. 
     FIG. 4 presents a cross-sectional view of the scroll apparatus 20 taken at an angular location at which there are five chambers C 1  through C 5 , as shown in FIG. 3. Each of the chambers generates an axial separating force a and a radial separating force s. For example, chamber C 1  would generate force vector a 1  as an axial separating force upon the end plate 82 tending to separate the drive scroll end plate 82 from the idler scroll end plate 102, and force vector s 1 , a radial separation force, would act upon the scroll wrap 80 tending to cause a separation from the second scroll wrap 100. Both force vectors a 1  and s 1  would tend to cause a turning or tipping of the first scroll member 76 perpendicular to the axis of rotation of the scroll member. The total axial separation force a is equal to the vector sum al plus a 2  plus a 3  plus a 4  plus a 5  and the net radial separation force s equals the vector sum s 1  plus s 2  plus s 3  plus s 4  plus s 5 . The net separation force is offset from the axis of rotation of the first scroll member 76. As a result, an instantaneous tipping moment mt is produced. The moment m t  acts upon the scroll member 76 to produce a tipping or nutation shown as angle delta d . Because the chambers are disposed at the same radial and angular location and the fluid forces are the same, but the axes of the scroll members 76 and 78 are offset, the forces in each chamber act to produce a tipping moment m t  for each scroll member 76 and 78. Therefore, the forces in chambers C1 through C5 act to produce a tipping or nutation of the scroll member 78 shown as angle delta i , which may differ from the angle delta d  produced in the scroll member 76 due to differences in the number, types, and sizes of bearing supporting the respective scroll member shafts and other constraints on the respective scroll member end plates. The scroll wraps 80 and 100 will typically separate when delta i  and delta d  differ. 
     This calculation must be repeated for each angular point of rotation for the respective scroll members 76 and 78. As shown in FIG. 4, an axial biasing force Fd is provided upon the drive scroll member 76 and an axial biasing force Fi is provided upon the idler scroll member 78 by the axial biasing means. The force Fd must be sufficient to exceed the axial separation force a d , and simultaneously must exceed the moment m t  with a moment M e  produced by the product of (Fd-a d ) times the available or effective contact radius of the scroll tips with the opposing scroll end plate, in order to prevent tipping of the scroll member end plate 82 at any given radial position. Where the force a d  exceeds the force (Fd-a d ), due to the tipping moment m t  will occur. Tipping may even occur when the force a is less than the force Fd where either the force Fd or the contact radius is insufficient to provide a counteracting moment. The force Fi is similar in nature. 
     FIG. 5 shows an analysis of the instantaneous tipping moments acting upon one of the scroll members 76 or 78 during the rotation of the scroll member. Crank angle refers to the angular position of the respective scroll members from the position at which phi 0  occurs, being between 0° and 360° (full circle) on the horizontal axis of the diagram, while the vertical axis of the diagram discloses the moment experienced at each radial position and the axial contact force Fd minus a d  at each radial position. The curve representing the instantaneous moment at each radial position is roughly sinusoidal, as is the curve representing the axial contact force. 
     FIG. 6 shows the instantaneous moments acting upon one of the scroll members 76 or 78 during the rotation of the scroll members with the Cg of the mass m 1  disposed at various radii r 1  at a given phi 1 , or the Cg of the mass m 2  disposed at various radii r 2  at a given phi 2 . For simplicity, the subscript is deleted, since the Figure is representative of conditions which may occur in either scroll member 76 or 78. Those radii represented include r=0, r=1 unit, r=2 units and r=3 units, where both phi and mass are constant. As noted above, M e  represents the moment produced by the product of (F d  -a d ) times the available or effective contact radius of the scroll tips with the opposing scroll end plate. 
     Those skilled in the art will recognize that specific unit measurements are not given in FIG. 6 since the invention is applicable to scroll apparatus of any size, and further because the FIG. 6 is intended to be representative of the results obtained generally by the application of the mass 160 or 162 to the scroll apparatus and is not therefore to be taken as limited to a specific case. Suitable specific unit measurements would include multiples of tens of inchs or centimeters, and multiples of inches or centimeters. 
     It will be observed that the graph representing the instanteous moments for r=0 produces the highest maximum moment at those crankangle positions where the available countering moment is minimal. The graph representing the instanteous moments for r=2 produces a lesser maximum moment. When r=3, the lowest maximum moment is produced in the exemplary apparatus at those crankangle positions where the available countering moment is minimal. It will be appreciated that these graphs are illustrative and are by way of example only, rather than limiting, since the actual angle phi and radius r selected for disposition of the moderating mass will vary for each scroll member to which the subject invention is applied, and the actual nutation observed in any scroll apparatus 20 depends upon the actual tipping moment at any angular position versus the available counteracting moment for preventing nutation. However, as exemplified, the radius r=3 is the preferred position for the placement of the moderating means, mass m, since the curve Me is not exceeded at any crankangle position, and the radius r=0 is the least desirable placement. 
     It will be appreciated that the mass m 1  and m 2  of masses 160 and 162 creates a mechanical dynamic imbalance of the scroll end plates 82 and 102 which, by the placement of the masses at predetermined locations on the respective end plates, creates a force which acts in opposition to and reduces the maximum tipping moment generated by the fluid forces acting on the scroll end plates 82 and 102. The moderating moment generated by the mass 160 and 162, which acts in opposition to the maximum tipping moment, is additive to the minimum moment of the scroll member generated by the mechanical components of the scroll member. Therefore, it is necessary to select the amount of the mass m 1  and m 2  of the masses 160 and 162 so that the necessary moderating moment is obtained without adding excessively to the minimum moment of the scroll member. 
     The method of reducing the moment of the scroll member by providing a moderating moment by mass-induced scroll imbalance includes the following steps: the instantaneous tipping moment acting upon a first scroll is determined for each angular position; the maximum tipping moment together with the angular or crankangle position or range of angular positions at which the maximum tipping moment acts is then determined; a moderating moment required to moderate the first scroll maximum tipping moment is determined, the amount m 1  of a first mass 160, an the radius r 1  and angular disposition phi 1  of such a first mass 160 to induce the desired moderating moment is determined; and the first mass 160 is applied to the first scroll member 82. This first mass 160 may be mechanically applied by welding or other means, or may be made integral with the first scroll member 76 at the time of manufacture. In order to further enhance the nutational stability of both scroll members in the scroll apparatus 20, a mass 162 may be applied to the second scroll member 78 by a method comprised simply of repeating the steps utilized to determine the mass and disposition of the mass 160 for the first scroll member 76. 
     Those skilled in the art will recognize that the use of the mass induced moment for enhancing the nutational stability of the co-rotating scroll apparatus 20 represents a substantial improvement in the art. The mass 160 and 162 may be determined by analytical methods, and involve no moving parts which require additional maintenance and increase the initial expense of the compressor assembly 20. Furthermore, the use of the masses 160 and 162, which creates a purposeful dynamic imbalance in their respective scroll members the effect of which is to create a tipping force which acts in opposition to the maximum tipping moments to which their respective scroll members would otherwise be subject to in operation, reduces the overall axial biasing force which must be applied to the scroll members to ensure that they do not separate, at any rotational position, as a result of the gas compression forces which exist therebetween in operation. This in turn reduces the frictional losses between the tip scroll wraps 80 and 100 and the end plates 82 and 102, respectively, which in turn reduces the power consumption of the scroll apparatus 20 for a given capacity, permitting the use of smaller and lighter motors 40. In all respects, therefore, the subject invention represents a substantial improvement which reduces the initial cost and improves the overall efficiency of the scroll apparatus 20. Furthermore, although the subject invention is exemplified in a scroll apparatus 20 useful in refrigeration system applications, it will be undoubtedly appreciated that the subject invention is useful in all applications of the co-rotational scroll apparatus 20, including pumps, expanders, fluid driven engines, and other applications, with like improvement in performance and reduction of expense. 
     Modifications to the preferred and alternate embodiments of the subject invention will be apparent to those skilled in the art within the scope of the claims that follow hereinbelow.