Scroll compressor with improved axial compliance
This application is a continuation of application Ser. No. 07/763,691 filed Sep. 23, 1991, now abandoned. 
    
    
     TECHNICAL FIELD 
     This invention relates to scroll compressors, and more particularly to improving axial compliance between scroll elements thereby achieving higher efficiency in scroll compressors. 
     BACKGROUND OF INVENTION 
     Scroll compressors have a wide range of applications where low to moderate compression ratios are desired, especially in the air conditioning and heat pump industries. This acceptance is attributed to high efficiency, fewer parts, and less noise and vibration when compared with competing compressors. A conventional scroll compressor includes a motor, which drives a shaft with an eccentric crank, causing orbiting motion of an orbiting scroll element. The orbiting scroll element has a scroll or spiral shaped protruding wrap, which interacts with a similarly shaped protruding wrap on a mating fixed element. Compression is achieved when the meshing coaction between the two protruding wraps shifts the gaseous fluid radially inward and simultaneously reduces the volume of the fluid. 
     However, internal leakage of pressurized fluid reduces the efficiency of scroll compressors. There are two types of leakage associated with scroll compressors, one is flank leakage, and the other is tip leakage. In both cases, the fluid in higher pressure pockets escapes through the gaps into lower pressure pockets. Flank leakage occurs when fluid from a pocket formed between the two protruding meshing wraps escapes at the flank surfaces where they come into contact with each other. Tip leakage occurs when fluid escapes between the end surface of the protruding wrap of each element and the base of the other element as they come into contact. Tip leakage is the more severe of the two because the effective total leakage path width for tip leakage is typically several times larger than that for flank leakage. Further, the compression process produces large axial loads that push the orbiting scroll element axially away from the fixed scroll element, thereby increasing the tip leakage. In addition to the axial forces driving orbiting scroll element away from the fixed scroll, there is also an overturning moment attempting to tip the orbiting scroll element out of the plane with the fixed scroll element. 
     This overturning moment results from the couple established between the non-axial component of the pressure forces generated within the compression pockets during the compression process and the reaction force thereof established between the shaft of the orbiting scroll element and its support bearings. 
     Since close-tolerance manufacturing techniques are not adequate to prevent the loss of pressure due to tip leakage, other methods have been developed. One approach is to utilize various types of tip seals, as described in U.S. Pat. Nos. 4,395,205; 4,411,605; 4,415,317; 4,416,597. The end surface of the protruding wrap of either scroll element is equipped with tip sealing means which reduce the tip leakage. Although this method is effective for sealing, it requires complicated manufacturing, increases friction, and raises costs. 
     Another approach to decrease tip leakage is to apply compensating back pressure to force mating elements together. Higher pressure fluid is purposely bled from the compression chamber through a vent port into a back chamber, which is typically a single, relatively large chamber located behind the orbiting scroll. This provides a body of pressurized fluid which pushes the orbiting element against the fixed element and thus, reduces the gap between the tips of the protruding scrolls and the bases of the elements. Reducing the gap minimizes the leakage of fluid, resulting in the increase of pressure in the compression chamber. 
     For example, U.S. Pat. Nos. 4,384,831; 4,600,369; 4,645,437; 4,696,630; and 4,861,245, each disclose a scroll compressor having such a back chamber. Commonly-assigned U.S. Pat. Nos. 4,992,032 and 4,993,928 also disclose scroll compressors using the back pressuring technique. As disclosed therein, rather than a single back chamber, two sealed pressure chambers, one at intermediate pressure and another at discharge pressure, are disposed behind the orbiting scroll element and are designed to counteract the gas compression forces within the compression chamber and to bias the orbiting scroll element toward the fixed scroll element. However, the prior art back pressuring technique is designed to overcome the highest overturning moment experienced during the orbiting cycle and leads to excessive thrust force over the remainder of the cycle. The large thrust force causes excessive friction between the two mating parts and results in reduced efficiency of the scroll compressors. 
     Additionally, U.S. Pat. No. 4,557,675 discloses a method of adjusting pressure in the back chamber by positioning pressure-equalizing ports so that the pressure vented into the back chamber varies with changes in operating conditions. However, the back pressure remains relatively constant during any given steady-state condition, thus, the change in pressure, as the operating conditions vary, is intended to overcome the highest overturning moment and axial force, resulting in excessive thrust force during the remainder of the cycle and causing excessive friction, thereby reducing the efficiency of the scroll compressor. 
     DISCLOSURE OF INVENTION 
     An object of the invention is to increase the efficiency of scroll compressors by reducing frictional forces between the scrolls. 
     According to the present invention, pressurized fluid is vented from the compression chamber into at least one dynamic back chamber through a port in the scroll element, so that the back pressure will vary on a sub-cycle basis. A dynamic back chamber, characterized by a relatively small volume of the chamber and a large flow area port for supplying pressure fluid thereto, is located behind the orbiting element. In accordance with this invention, an efficient means of counteracting the overturning moment without producing excessive friction forces may be achieved by varying the back pressure on a sub-cycle basis. 
     These and other objects, features, and advantages of the present invention will become more apparent in light of the detailed description of a best mode embodiment thereof, as illustrated in the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF DRAWING 
     FIG. 1 is a diagrammatic, side elevation view of a scroll compressor in accordance with the present invention; 
     FIG. 2 is a sectioned plan view illustrating the meshing of the protruding scroll wraps of the scroll compressor shown in FIG. 1 so as to form compression pockets therebetween; 
     FIG. 3 is an enlarged, partial, sectioned view of a portion of the scroll compressor of FIG. 1; 
     FIGS. 4a and 4b are exemplary graphs of overturning moment versus crank angle for two different operating envelope conditions, underpressure and overpressure, respectively; and 
     FIGS. 5a and 5b are exemplary graphs of the minimum compliance forces required to counter the overturning moments to FIGS. 4a 4b, respectively, and actual backpressure compliance forces produced in accordance with the present invention versus crank angle. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Referring now to FIGS. 1-3, a scroll compressor 10 includes a fixed scroll 11 which is engaged with an orbiting scroll 13. The orbiting scroll 13 is driven by a shaft 17 which is driven by motor 15 in orbital movement relative to the fixed scroll 11. Fluid compression is achieved as scroll wraps 18, 20 protruding from the orbiting scroll 13 and the fixed scroll 11, respectively, mesh to form a plurality of compression pockets 19 therebetween to trap volumes of fluid. This orbital action displaces the pockets of trapped fluid spirally inward while simultaneously reducing fluid volume of the pockets thereby compressing the fluid trapped therein. 
     During the compression process, pressure forces having axial and non-axial components are generated within the compression pockets. Referring to FIG. 3, the resultant axial force, F pa , tends to push the orbiting scroll 13 away from the fixed scroll 11 and the resultant tangential force, F pt , forms a couple with the reaction force, F br , thereto established between the hub of the orbiting scroll 13 and the support bearings 14 on shaft 17, which couple produces an overturning moment, M o , which tends to tip the orbiting scroll 13 relative to the fixed scroll 11. Due to the pressures created in the static and dynamic backpressure chambers, an axially directed resultant backpressure compliance force, F pr , is produced which acts substantially along the central axis of the drive shaft 17 and comprises the sum of the axial components of the distributed pressure forces, F ps  and F pd , produced in the static and dynamic backpressure chambers, respectively, and acting upon the back of the orbiting scroll to push the orbiting scroll 13 against the fixed scroll 11. This resultant back pressure force, although acting substantially along the central axis of the shaft 17, does not act through the center of mass of the orbiting scroll 13 mounted to the eccentric crank portion 17A of the shaft 17. There is also established a net reaction force, F nr , resulting from the net interaction of all axial pressure forces acting on the orbiting scroll, that is the axially directed resultant backpressure compliance force, F pr , and the opposed axially directed pressure force, F pa ,. This net reaction force acts as a thrust force on the orbiting scroll at a radial distance from the center of mass of the orbiting scroll, thereby creating a counteracting moment, M c , which acts in opposition to the overturning moment, M o . 
     As best seen in FIG. 3, a flow of pressurized fluid is bled through the ports 21, 23 into back chambers 25, 27, respectively. The fluid in these chambers produces back pressure which pushes the orbiting scroll 13 towards the fixed scroll 11 in order to reduce tip leakage and counteract overturning moment. However, the back pressure produced is not constant over the entire cycle. Instead, it varies during the cycle to follow the fluctuations in the overturning moment, which acts on the orbiting scroll 13 and causes it to tip with respect to the fixed scroll 11. Thus, the back pressure created is just enough to counteract the overturning moment. When the overturning moment is high, greater back pressure is available to hold the orbiting scroll in place to avoid leakage. When the overturning moment is low, the back pressure is also less and thus, does not cause excessive friction loss. This effect is attained by providing at least one dynamic chamber in which the pressure fluctuates in proportion to the overturning moment. 
     In the embodiment shown, there are two ports 21, 23 and two corresponding chambers 25, 27. Port 23 supplies pressurized fluid into the static chamber 27. Port 21 supplies pressurized fluid into dynamic chamber 25. The distinction between the two is that static chamber has a relatively constant fluid pressure throughout the entire cycle, while the dynamic chamber has widely varying fluid pressure during the cycle. The static port/chamber combination has a small port diameter and a large chamber volume. The dimensions are selected in such a way as to produce sufficient damping so that pressure is nearly constant throughout the cycle. 
     The variation of pressure on a sub-cycle basis in the dynamic chamber is attained by properly sizing the port diameter and chamber volume parameters relative to each other. The dynamic port/chamber pair has a large diameter port 21 and small chamber volume 25. The dimensions are selected in such a way as to produce very little damping so that the pressure in the dynamic chamber follows the compression process. This achieves the pressure variation on a sub-cycle basis. 
     It has been found that in order to maintain substantially constant pressure in the static chamber, the ratio of port diameter to the cubed root of chamber volume should be relatively small. In order to provide widely varying pressure in the dynamic chamber the ratio should be relatively large. For example, when a compressor designed with a static chamber having the ratio of 0.05 and dynamic chamber having a ratio of 0.22 was tested, it exhibits a roughly 45% reduction in net axial force. 
     In FIGS. 4a and 4b, the variation of overturning moment versus crank angle is illustrated over one orbiting cycle for two different operating conditions, under-pressure and over-pressure, respectively. The crank angle is commonly known in the art to refer to the circumferential displacement, measured in degrees, of a radial reference line on the orbiting scroll from a radial reference line on the fixed scroll. The overturning moment produced by the couple formed by the resultant tangential pressure force and the bearing reaction force various substantially under both operating conditions. Referring now to FIGS. 5a and 5b, curve A represents the minimum compliance force required to be exerted axially against the back side of the orbiting scroll to overcome the overturning moment illustrated in FIGS. 4a and 4b, respectively, at each crank angle over one orbiting cycle, that is to prevent the overturning moment from tipping the orbiting scroll relative to the fixed scroll. Curve B represents the backpressure compliance force exerted axially against the back side of the orbiting scroll of a scroll compressor embodying a static backpressure and a dynamic backpressure for these two operating conditions. This backpressure compliance force is the sum of the axially directed pressure forces, F pg  and F pd , produced in the static and dynamic backpressure chambers, respectively. As the backpressure in the static chamber 27 remains substantially constant over an orbiting cycle and the backpressure in the dynamic chamber 25 varies in proportion to the variation of the pressure within the compression pockets 19 over an orbiting cycle, the backpressure compliance force represented by curve B closely approximates the minimum required backpressure force necessary to overcome the overturning moment at each crank angle over an orbiting cycle, thereby counteracting the overturning moment without producing excessive friction forces and a consequent reduction in operating efficiency. 
     Although the embodiment illustrated has one dynamic and one static chamber/port combination, other combinations are possible. This invention encompasses any number of dynamic chamber/port combinations that is one or more, with or without any number of static chambers. Since the total back pressure force on the scroll is the sum of the forces generated by the constant pressure in the static chamber and the varying pressure in the dynamic chamber, the total back pressure varies over the orbiting cycle instead of remaining constant, as in the prior art. 
     Also, one port may lead to more than one chamber and vice-versa, more than one port may lead into one chamber, as long as the appropriate ratios of effective port diameter/cubed root of effective chamber volume are maintained. Another variation that may yield substantially similar results is that back pressure may be applied to the fixed scroll, as opposed to the orbiting scroll, wherein the fixed scroll is able to move axially. Although the exact position of ports is not critical to this invention and may depend on characteristics of each compressor, the port location selection should utilize the pressure variation inside the compression chamber in order to produce sufficient pressure in the back chamber. 
     Although the invention has been shown and described with respect to a best mode embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions, and additions in the form and detail thereof may be made therein without departing from the spirit and scope of the invention.