Abstract:
A scroll compressor is provided with a generally spiral wrap having an abradable or conformable coating on radially inner portions of its tip. The coating is not applied to radially outer portions. During operation of the scroll compressor, the coating will eliminate tip gap leakage adjacent radially inner, higher discharge pressure portions of the compression process. The outer, non-coated portions will bear the axial load, and ensure the coating will not wear away to an undue amount.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to a scroll compressor utilizing an abradable coating over a portion of its wrap only such that the abradable coating will provide a tight tip seal, but the uncoated portions of the wrap will provide a load-bearing surface. 
   Scroll compressors are becoming widely utilized in refrigerant compression applications. In a scroll compressor, first and second scroll members each include a base and a generally spiral wrap extending from the base. The wraps interfit to define compression chambers. One of the two wraps is caused to orbit relative to the other, and as the two orbit, the compression chambers are decreased in volume, thereby compressing an entrapped refrigerant. 
   In scroll compressors, the tip of each wrap is ideally in contact with the base of the opposed scroll member. A prior art scroll compressor is shown in FIG.  1 . In prior art scroll compressor  20 , the orbiting scroll  22  is placed into operable relationship with the non-orbiting scroll  24 . The orbiting scroll  22  has a wrap  26  extending from its base  27 . The wrap  26  contacts the opposed base  29  of the non-orbiting scroll  24 . The wrap  28  from the non-orbiting scroll  24  contacts the base  27 . Ideally, the tips of the wraps are in abutting contact with the opposed bases. 
   However, as shown in  FIG. 2 , it is often the case that local height variation along the wrap length may prevent sealing contact with the floor of the opposed base. The leakage sensitivity to a given gap is greater near the center of the wrap in that pressure will be higher adjacent the center. This leakage over the tip of the scroll compressor and through the gap  23  is undesirable and reduces efficiency. 
   Scroll compressors have been proposed wherein a coating has been applied along the entire wrap. However, this has undesirable characteristics. 
   SUMMARY OF THE INVENTION 
   In the disclosed embodiment of this invention, an abradable or conformable coating is applied to a scroll wrap tip only adjacent radially inner portions of the wrap. Radially outer portions of the wrap remain uncoated, and provide a load-bearing surface. In this way, the coating is not subject to bearing the load, but still eliminates the tip gap problem, and in particular at the more central portions of the compression process. 
   In a disclosed embodiment, a step is formed into the wrap adjacent its center such that it is slightly shorter than the outer portions. Outer portions of the wrap remain uncoated. A coating is placed within the stepped area such that the surface of the coating is slightly taller than the outer portions of the wrap. As the scroll begins to operate, the abradable or conformable coating abrades or flows away leaving a smooth surface which will eliminate any tip gap adjacent the center portion of the wrap. As the coating abrades or flows away, the uncoated outer portions come into contact and provide a load-bearing surface so that the coating is not entirely abraded or worn 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  shows a prior art scroll compressor. 
       FIG. 2  shows a problem in the prior art scroll compressor. 
       FIG. 3  shows an inventive scroll wrap. 
       FIG. 4  shows a first step in forming the inventive scroll wrap. 
       FIG. 5  shows the final configuration for the inventive scroll wrap. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A scroll  122  is illustrated in FIG.  3 . The wrap  26  has an outer portion  30  which is formed of a metal such as iron, and is as typically known in the current art. A radially inner center portion  31  is provided with an abradable or conformable coating. While any appropriate coating may be applied, 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  122  may be on either the orbiting or non-orbiting scroll, or both. 
   The scroll compressor has a load-bearing surface borne by the metal portion  30  at its uncoated outer peripheral locations. The abradable or conformable coating provides a tight tolerance fit between the tip and the opposed base wall adjacent its central portion. The problem shown in  FIG. 2  becomes particularly pronounced at radially inner locations where the pressure between the adjacent chambers H and L is most pronounced. As the pressure of the high pressure chamber H increases, the amount of leakage through the gap  23  to the low pressure chamber L increases. Thus, having the coating at the radially inner or central portions eliminates the leakage at the most effective locations. 
   As shown in  FIG. 4 , a step  34  is cut into the wrap. A portion  35  of the wrap remains formed of the metal adjacent the central portion. A coating  38  is deposited into the stepped portion. As can be seen at  100 , the coating surface  32  would initially extend beyond the nominal height of the outer portion  30 . 
   During run-in, the compressor begins to operate, this abradable or conformable coating  38  will wear away such that it eventually has the final shape shown in  FIG. 5  at  33 . After run-in, this coating would thus eliminate the tip gap problem as shown in FIG.  2 . However, as the conformable coating is deformed or worn away, the uncoated harder metal outer portions  30 , come into contact and provide a load-bearing surface, and thus complete wear-out of the coating  38  will be prevented. 
   The coatings that are most suited for this application could be generically described as a coating which 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 tip is brought into contact with the opposed mating scroll floor, the coating will move to take on the shape defined by the floor, 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 member, will take on a shape defined by the mating member. The coating will typically have a bulk hardness or strength which is somewhat less than that of the mating floor of the opposed scroll member, and for that matter, also of the material utilized to form the scroll tip 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 that 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 tip. 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 that wears or flakes away under the influence of pressure and/or shape from the mating floor 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 that will change their shape upon the influence of the opposed mating floor of the opposed scroll member. In that sense, the material of the coating will move upon contact with the floor of the mating scroll member. This “changing” and “movement” can be abrasion or the type of movement without abrasion provided by a conformable coating. 
   While a preferred embodiment of this invention has been disclosed, 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.