Abstract:
An abradable coating is formed on portions of the flank wall of the wraps of a scroll compressor. The radially outer portions of the flank wall are left uncoated, and can bear a force between the scroll wraps. In this manner, the load is not transferred through a coated portion, but rather through the uncoated portion. As the scroll wraps move relative to each other at run-in, the abradable coating wears away leaving a tight fit between the flank walls at the radially inner locations.

Description:
BACKGROUND OF THE INVENTION 
   A scroll compressor has at least one of its wrap walls provided with an abradable coating to eliminate wrap leakage between opposed walls of the orbiting and non-orbiting scroll. The coating is only provided at radially inner locations, with radially outer locations being left to bear the load between the orbiting and non-orbiting scrolls. 
   Scroll compressors are becoming widely utilized in refrigerant compression applications. In a scroll compressor, a pair of interfitting scroll members each have a base and a generally spiral wrap extending from its base. The wraps interfit to define compression chambers. One of the two scrolls is caused to orbit relative to the other. As the orbiting movement occurs, the wrap walls are brought into contact, and the wrap tips are brought into contact with the floor of the base of the opposed scroll. As the orbiting movement occurs, the compression chambers are reduced in volume, thus compressing the entrapped refrigerant. 
   A scroll compressor as known in the prior art is shown at  20  in  FIG. 1 . The orbiting scroll  22  is placed adjacent the non-orbiting scroll  24 . The orbiting scroll  22  has a generally spiral wrap  26  extending from the floor  27  of its base. The non-orbiting scroll  24  has its own generally spiral wrap  28  extending from its base  29 . 
   As shown in  FIG. 2 , this prior art compressor has contact between the flank walls of the scroll wraps  26  and  28  to define the compression chambers. As an example, one intended point of contact between the wraps  26  and  28  is shown at  30  and  32 . These points of contact define compression chambers such as compression chambers  100  and  102 . Other points of contact are shown at C. However, as shown in  FIG. 3 , there have sometimes been gaps such as gap  34  between the points  30  and  32 . When this occurs, there is potential leakage between chambers  100  and  102  which will reduce the efficiency of the compression process. The gap  34  can be caused due to machining variations, thermal distortion, or other problems. Generally, such scroll compressors are “radially compliant,” allowing one of the wraps to move into abutting contact with the other at at least one point. However, as can be appreciated from  FIG. 2 , it would be desirable to have contact at several points C. It has been somewhat difficult due to the tolerance issues, etc. mentioned above, to ensure that each of the contact points will meet during operation. Again, this may result in a decrease in efficiency. 
   While coatings have been proposed for the wraps of scroll compressors, they have generally been along the entire length of the wrap. To have the coating along the entire length would have undesirable characteristics. 
   SUMMARY OF THE INVENTION 
   In the disclosed embodiment of this invention, a coating is provided adjacent radially inner or central portions of the wrap of at least one of the two scroll members. The coating is preferably made of an abradable or conformable material and thick enough such that after run-in of the scroll compressor, the coating will remain to eliminate gaps such as the prior art gap ( FIG. 3 ,  34 ) by ensuring that the coating will only be worn away as would be necessary to adjust to the particular tolerances, etc. for the particular scroll elements. 
   Preferably, the coating is formed at a location spaced radially inward somewhat from the beginning of the wrap. The radially outer portions of the wrap will bear the radial load between the two wrap members. It would be undesirable to have this load carried by the coating entirely, since the coating may then wear away. 
   These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view through a prior art scroll compressor. 
       FIG. 2  shows a problem with the prior art scroll compressor. 
       FIG. 3  is an enlarged view of a portion of  FIG. 2  showing the prior art problem. 
       FIG. 4  is a first step in forming an inventive scroll compressor. 
       FIG. 5  shows the final shape of the scroll compressor provided by this invention. 
       FIG. 6  shows another embodiment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A wrap  126  is shown in  FIG. 4 . It should be understood that the present invention may preferably be utilized on the wraps of both the orbiting and non-orbiting scroll. It is within the scope of this invention that only one of the two scrolls carry the coating. As shown, the wrap has a radially outer portion  36 , which preferably extends for at least 360°. Notably, and by comparing  FIG. 4  to  FIG. 2 , it is clear that a portion of the uncoated portions  36  will include some of the contact points C. In this manner, the load can be born by these radially outer contact points. This portion is left uncoated, and formed of the metal typically utilized to form scroll wraps. A ditch begins at portion  38 , and has a depth  39  extending to an end point  42 . A coating  40  is placed within the ditch  39 . Notably, there is an uncoated portion  44  radially inwardly of the end point  42  of the ditch. Notably, it is possible that the inner end is also coated. The scroll compressor is provided with this coating, with the coating being thick enough such that it will allow the scroll compressor to “run-in” eliminating any gaps such as the gaps shown in  FIG. 3  which may be due to machining variations, tolerances, etc. It should be understood that the problem shown, for example, in  FIG. 3 , is most pronounced at radially inner locations along the wraps wherein the compressed refrigerant is reaching higher pressures. While any appropriate coating may be approved, examples of appropriate abradable or conformable coatings include iron phosphate coatings, magnesium phosphate coatings, nickel polymer amalgams, and other materials that abrade or yield plastically when a force is applied. The coated scroll  130  may be on either the orbiting or non-orbiting scroll, or both. 
   During run-in, the coating  40  will wear away to ideally match the desired shape for the particular scroll compressor. 
   The uncoated outer portion  36  will bear the radial load of force between the two scroll members, and thus the radial load will not be born by the coated portion. 
     FIG. 5  shows the wrap after run-in. The coating will now have a surface  50  that has moved away from the surface  40  as shown in  FIG. 4 . Thus, with this coating, the seal leakage such as shown in  FIG. 3  will be eliminated. 
     FIG. 6  shows another embodiment scroll member  200  wherein the coating material  202  is formed in a ditch  204  in the radially outer face of the scroll wrap. In such an embodiment, the coating material  202  preferably extends to the tip  206  of the wrap. Notably, the coating material does not begin for at least 360° measured from an outermost edge  207  of the wrap. 
   The coatings that are most suited for this application could be generically described as a coating that will move when the coated scroll member is brought into contact with the mating surface. For purposes of this application, this would mean that when the scroll wrap is brought into contact with the opposed mating scroll wrap, the coating will move to take on the shape defined by the opposed wrap, and eliminate the gap. The coating could be a “conformable” coating that under the influence of pressure or relative motion from the opposed mating scroll wrap, will take on a shape defined by the mating wrap. The coating will typically have a bulk hardness or strength which is somewhat less than that of the mating wrap of the opposed scroll member, and for that matter, also of the material utilized to form the scroll wrap on which it is deposited. Composite coatings may be utilized which could be made up of two or more mechanically bonded components. As an example, a carbon fiberfill resin, although this particular example would be a somewhat unlikely example. In such a composite, one or more of the coating components may have a hardness or strength equal to or greater than the scroll members, but the aggregate of the two would result in the coating having a hardness or strength which is less than that of the scroll members. 
   Another example of a conformable coating would be an extrudable or deformable coating. This is a type of conformable coating which when brought into contact with the mating surface and under the influence of pressure and/or relative motion, plastically extrudes or flows until it takes on a shape defined by the mating surface. This type of coating typically does not wear or flake away. The coating material remains attached to the coated wrap. Such coatings are often of a composite type with a harder matrix material to provide structural integrity and a softer filler component to lower the bulk hardness and allow the material to flow. Such materials have been utilized, as an example, in screw compressors wherein a known coating was a nickel-polymer aggregate. In such a coating, a nickel “foam” is filled with a soft polymer material. Open metal foams may be desirable for this application. However, such forms may also have some difficulty in that parts of the metal foam may sometimes break away, which could result in undesirable abrasive debris. In the known nickel-polymer aggregate, the polymer nodules provide an internal hydrostatic-type support to prevent the metal matrix from bending too much locally, which provides the benefit of good bond strength holding the deformed metal in place. 
   Another type of coating within the scope of this invention is an abradable coating. This is a type of coating which wears or flakes away under the influence of pressure and/or shape from the mating wrap of the opposed scroll member. Such coatings tend to be soft, and if they are formed of a composite, all of the components are typically soft. The wear debris will circulate through the rest of the compressor mechanism, and it would be undesirable to have an unduly abrasive “grit” provided by such an abradable coating. 
   For purposes of this application, the above coatings are generally referred to as coatings which will change their shape upon the influence of the opposed mating wrap of the opposed scroll member. In that sense, the material of the coating will move upon contact with the wrap of the mating scroll member. This “changing” and “movement” can be abrasion or the type of movement without abrasion provided by a conformable coating. 
   Although preferred embodiments of this invention have been shown, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.