Patent Abstract:
A compressor has a housing assembly and at least one rotor held by the housing assembly for rotation about a rotor axis. The rotor has a first face and a first housing element has a second face in facing spaced-apart relation to the first face of the rotor. One or more spacer elements are positioned from the first housing element. The spacer elements are machined. A coating is applied over the first surface around the one or more spacer elements.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional and continuation-in-part of Ser. No. 10/828,404, now U.S. Pat. No. 7,318,275, filed Apr. 19, 2004, and entitled “Method of Remanufacturing a Compressor” which is a divisional of Ser. No. 10/331,793, now U.S. Pat. No. 6,739,851, filed Dec. 30, 2002, and entitled “Coated End Wall and Method of Manufacture”, the disclosures of which are incorporated by reference in their entireties herein as if set forth at length. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    This invention relates to compressors, and more particularly to screw compressors. 
         [0003]    Screw-type compressors are commonly used in refrigeration applications. In such a compressor, intermeshed male and female lobed rotors or screws are driven about their axes to pump the refrigerant from a low pressure inlet end to a high pressure outlet or discharge end. The rotors are typically supported by bearings on inlet and outlet sides of their lobed working portions. 
         [0004]    The clearance between the discharge end faces of the rotors and the adjacent housing surface influences compressor efficiency. A tight or small clearance improves efficiency by reducing internal leakage. Maintaining a tight clearance may require precision machining and alignment of these surfaces. A tight clearance, however, risks metal-to-metal contact between the surfaces which may cause damage. Accordingly, for controlling leakage while maintaining metal-to-metal clearance, it is known to utilize a relatively soft coating on the housing surface to partially fill the metal-to-metal clearance. Should a rotor contact the coating, the coating will be conformed and/or abraded without substantial damage to metal components or performance. Various plastically conformable coatings are known, including, iron phosphate, magnesium phosphate, nickel polymer amalgams, nickel zinc alloys, aluminum silicon alloys with polyester, and aluminum silicon alloys with polymethylmethacrylate (PMMA). These may be applied by appropriate methods, including, for example, thermal spraying, physical vapor deposition (PVD), chemical vapor deposition (CVD), and aqueous deposition. 
         [0005]    In an exemplary method of manufacture of such a compressor, the discharge end housing surface (e.g., of an outlet casing element of the housing assembly) is precision machined. The coating is then applied and the coating is machined to a desired final thickness. In this example, the precise thickness is required to provide precision in a subsequent end clearance setting process. In that process, the rotors are assembled and placed in a rotor housing portion of the housing assembly. The outlet casing is installed as are the bearings on the discharge end of the rotor shafts. Shims are inserted to cooperate with the thrust and radial bearings to constrain the longitudinal movement of the rotors relative to the outlet casing. The rotors are pulled against the outlet casing to zero a measurement tool. The rotors are then pushed away until restrained by their respective thrust bearings. The displacement is measured and this determines the clearance upon final assembly. If each measured clearance is within specified limits, the compressor may be further assembled. If not, for any rotor outside the limits, a different shim combination may be selected to bring the measured clearance more in line with the specified clearance and the process repeated. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    A compressor has a housing assembly and at least one rotor held by the housing assembly for rotation about a rotor axis. The rotor has a first face and a first housing element has a second face in facing spaced-apart relation to the first face of the rotor. The housing has a coating on the second face and a plurality of inserts protruding from the second face into the coating. 
         [0007]    Advantageously, the housing is made of a first material and the inserts consist essentially of a material that is more malleable than the first material. 
         [0008]    Another aspect of the invention involves a method of manufacture, remanufacture, or repair of a compressor. The compressor has a rotor with a working portion having a first end face. A housing assembly carries the rotor for rotation about a rotor axis. The housing assembly has a first housing element having a first surface facing the first end face. The method includes positioning one or more spacer elements from the first housing element. The one or more spacer elements are machined. A coating is applied over the first surface around the one or more spacer elements. 
         [0009]    In various implementations, there may be a plurality of such spacer elements (e.g., between three and five). The machining may provide coplanarity of first end surfaces of the spacer elements. The coating may be plastically deformed to a thickness associated with a height of the spacer elements (e.g., above the housing first surface). The thickness may be between 40 and 250 μm. The plastic deformation may consist essentially of compressing (e.g., with the rotor or with a flat element). The positioning may comprise press fitting. Old spacer elements may be removed before inserting the spacer elements. The rotor may be a screw-type male rotor and the compressor may further include at least one screw-type female rotor and meshed with the male rotor. 
         [0010]    Another aspect of the invention involves a method of manufacture, remanufacture, or repair wherein a coating is applied over a housing first surface around a number of spacers protruding from the housing. The coating is plastically deformed by compressing. 
         [0011]    Another aspect of the invention involves a method of manufacture, remanufacture, or repair including one or more steps for providing at least one spacer element protruding from a housing first element. A coating is applied in one or steps over a first surface of the first housing element. The applied coating is precompressed in one or more steps. 
         [0012]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0013]      FIG. 1  is a partially schematic longitudinal sectional view of a compressor. 
           [0014]      FIG. 2  is an enlarged view of a portion of the compressor of  FIG. 1 . 
           [0015]      FIG. 3  is an enlarged view of a portion of the compressor of  FIG. 2 . 
           [0016]      FIG. 4  is an end view of a female rotor working portion. 
           [0017]      FIG. 5  is a longitudinal sectional view of an outlet casting. 
           [0018]      FIG. 6  is a view of the casting of  FIG. 5  after a recess machining. 
           [0019]      FIG. 7  is a view of the casting of  FIG. 6  after a coating. 
           [0020]      FIG. 8  is a view of the casting of  FIG. 7  after deformation of the coating. 
           [0021]      FIG. 9  is an end view of the casting of  FIG. 8 . 
       
    
    
       [0022]    Like reference numbers and designations in the various drawings indicate like elements. 
       DETAILED DESCRIPTION  
       [0023]    The invention relates to compressors and methods for manufacture, remanufacture and/or repair. Spacer elements are associated with the application of a coating to one or more select surfaces of the compressor to improve such manufacture, remanufacture and/or repair.  FIG. 1  shows a compressor  20  having a housing assembly  22  containing a motor  24  driving three rotors  26 ,  28 , and  30  having respective central longitudinal axes  500 ,  502 , and  504 . In the exemplary embodiment, the rotor  26  is centrally positioned within the compressor and has a male lobed body or working portion  32  enmeshed with female lobed bodies or working portions  34  and  36  of the female rotors  28  and  30 . Each rotor includes shaft portions (e.g., stubs  40 ,  41 ;  42 ,  43 ; and  44 ,  45  ( FIG. 2 ) unitarily formed with the associated working portion  32 ;  34 ; and  36 ) extending from first and second ends of the working portion. Each of these shaft stubs is mounted to the housing by one or more bearing assemblies for rotation about the associated rotor axis. 
         [0024]    In the exemplary embodiment, the motor is an electric motor having a rotor  50  and a stator  52 . A distal portion  54  of the first shaft stub  40  of the male rotor  26  extends within the stator  52  and is secured thereto so as to permit the motor  24  to drive the male rotor  26  about the axis  500 . When so driven in an operative first direction about the axis  500 , the male rotor drives the female rotors in opposite directions about their axes  502  and  504 . The resulting enmeshed rotation of the rotor working portions tends to drive fluid from a first (inlet) end plenum  56  to a second (outlet/discharge) end plenum  58  while compressing such fluid. This flow defines downstream and upstream directions. The exemplary housing assembly  22  includes a rotor housing  60  having a transverse web  62  in which the rotor inlet end shaft stubs are mounted via appropriate bearings, seals and the like. The rotor housing  60  extends upstream from the web to substantially contain and surround the rotor working portions. The rotor housing  60  extends upstream to mate with a motor casing  64  which cooperates with the rotor housing to support and contain the motor  24 . At its downstream end, the rotor housing  60  mates with an outlet casing  70 . For each of the rotors, the outlet casing has a bearing compartment carrying a series of bearing assemblies (described below) for rotatably mounting the downstream (outlet/discharge end) shaft stub of such rotor. The outlet casing further includes an upstream-facing end surface  72  ( FIG. 2 ) in close facing proximity to the discharge end faces (surfaces) of the rotor working portions. A bearing cover plate  78  is centrally mounted to the outlet casing to cover the bearing compartments. A discharge housing  80  ( FIG. 1 ) is mounted surrounding the bearing cover plate. Exemplary rotor and housing materials are metals. Exemplary housing components are made of gray iron. Exemplary rotors are made of ductile iron and/or steel. 
         [0025]      FIG. 2  shows further details of the mounting of the outlet end shaft stubs of the male and female rotors. Aligned in an inlet-to-outlet direction, the male rotor has a radial bearing  90 , a thrust bearing  92 , and a counterthrust bearing  94 . Along the shaft stub between the bearing  90  and the discharge end face  100  of the rotor working portion, a floating bushing seal  102  is carried by the outlet casing to engage the shaft and an axial seal  104  is carried by the outlet casing to engage the face  100 . The clearance between the surface  72  and the face  100  is determined by the cooperation of the bearings  90 ,  92 , and  94  along with any spaces and/or shims. A rotor cap  112 , secured to the end of the shaft stub, bears against the outlet end rim of the inner race of the third bearing  94  to capture the sandwich of the three inner races. A bearing retainer  114  has an inlet end rim engaging a preload spring  116  which in turn engages the outer race of the third bearing  94  and an outlet end rim engaging the bearing cover plate  78 . 
         [0026]    The outlet end shaft stub of each female rotor has, aligned in an inlet-to-outlet direction a radial bearing  120 , a thrust bearing  122 , and a counterthrust bearing  124 . A floating bushing seal  126  engages the shaft in a reduced diameter base portion of the bearing compartment. At its inlet end rim, the inner race of the bearing  120  contacts a shoulder of the shaft stub. A rotor cap  140 , secured to the end of the shaft stub, bears against the outlet end rim of the inner race of the bearing  124  to capture the sandwich of three inner races. A bearing retainer  142  has an inlet end rim engaging the outer race of the bearing  124  and an outlet end rim engaging a preload spring  143  which in turn engages the bearing cover plate. 
         [0027]      FIG. 3  further shows, in exaggerated thickness, a coating  200  on the surface  72  and a plurality of pins  220  mounted in bores  222  in the outlet casing and protruding from the surface  72  to extend into the coating. In the illustrated exemplary embodiment, four of the pins lie along the common plane of the rotor axes, whereas others are similarly oriented but lie away from the plane. Of these four pins, each of the outboard pins is associated with one of the female rotors and is positioned with its inlet end face  224  in close facing proximity to an area swept by the portion of the outlet end surface  118  that lies along the female rotor lobes. Each of the inboard pins is similarly positioned relative to one of the female rotors but is also positioned in an area swept by the end surface  100  of the male rotor along its lobes as shown in further detail in  FIG. 3 . 
         [0028]      FIG. 3  further identifies a pin length L 1 , a pin diameter D 1 , a coating thickness T 1 , an overall metal-to-metal clearance T 2 , and a metal-to-coating clearance T 3 . 
         [0029]      FIG. 4  shows an exemplary outlet end surface (face)  118  of a female rotor. The face includes portions  250  defined by the ends of the plurality of lobes and a central continuous annular portion  252  inboard of the lobe roots. In the illustrated embodiment, at the outlet end surface, the shaft stub has a diameter D 2 , the central portion  252  has a root diameter D 3  and the lobes have an outside diameter D 4 . 
         [0030]    In an alternate pin arrangement each pin associated with the female rotor is positioned to fall entirely under the root diameter D 3 . This permits a minimal number of pins as it guarantees pins will be aligned with the end surface regardless of rotor orientation. Although as few as one pin may be used, three are advantageous for purposes of precise orientation during the clearance setting process. If the pins were entirely positioned to fall between the root diameter D 3  and outside diameter D 4 , then, if it is desired that contact be assured irrespective of orientation during the clearance setting procedure, either particularly broad pins would have to be used (e.g., pins with large D 1  or having sections like an annular segment) or a greater number of pins would have to be used. 
         [0031]    In an exemplary method of manufacture, the pins are installed and their ends machined to provide the desired exposure (e.g., to T 1 ) in the same manufacturing station wherein the surface  72  is machined. The coating is then applied to a thickness of at least T 1 . A flat or other plate may then be pressed down atop the coating until stopped by engagement with the pin end face  224 . The compression advantageously plastically deforms the coating so that, when the plate and compressive forces are removed, the coating will retain a uniform thickness of T 1  coincident with or just slightly greater than the pin exposure. Alternatively, the rotor end faces could be used to plastically deform the coating by pulling the rotors into the coating until stopped by engagement with the pin end faces  224 . This method may be less advantageous as the interlobe area would leave portions of the coating uncompressed unless the rotors were rotated and the process repeated. 
         [0032]    Exemplary material for the pins is brass. Other materials, such as aluminum, bronze, or engineering plastics may alternatively be used. As described below, the pin material is advantageously softer and more malleable or otherwise deformable than that of the rotor so that, upon any rotor-to-pin contact the rotor will remain essentially undamaged, potentially sacrificing the pins. 
         [0033]    Advantageously the coating is of a conformable coating material as are known in the art (e.g., as described above) or may yet be developed. As applied, the coating may have an exemplary thickness between 30 and 500 μm. After initial compression, the exemplary thickness T 1  may well be between 20 and 300 μm. More preferably, such thickness may be between 40 and 250 μm. The exemplary metal-to-coating clearance T 2  may well be between 5 and 100 μm, more preferably such clearance T 2  may be between 10 and 20 μm, leaving a preferred metal-to-metal clearance T 3  between 50 and 270 μm. Exemplary coating processes are described above. Among alternate coating processes are application of pre-formed coating layers (e.g., a peel &amp; stick product with pressure-sensitive adhesive). 
         [0034]      FIGS. 5-9  show an example of an alternate implementation as a retrofit/modification of an existing compressor. Specifically,  FIG. 5  shows an existing/baseline outlet casting  70 . For example, the casting  70  may be removed from a compressor in service. Alternatively, the casting may be new (even including in an intermediate stage of manufacture). The exemplary outlet casting  70  is shown with pre-formed bearing compartments, mounting bores, and porting. The casting  70  may be fixtured and a recess/trough  300  ( FIG. 6 ) machined (e.g., with an end mill or other tool) in the initial end surface  72 . The machining leaves the trough having a base surface portion  72 ′ recessed below the intact surface  72  by a depth D. In a subsequent stage, a coating  320  ( FIG. 7 ) is applied in the recess/trough  300  to an initial thickness T 4 , leaving the exposed surface  322  of the coating proud of the surface  72 . Prior to application of the coating  320 , various features may be masked (e.g., the bearing compartments, ports, and intact portion of the surface  72  surrounding the recess). 
         [0035]    After coating application, the coating  320  may be compressed.  FIG. 8  shows a flat plate  330  compressing the coating so that the surface  322  becomes flush with the intact surface  72 . A portion of the plate  330  laterally beyond the coating may register with the intact surface  72  to ensure precise deformed coating thickness. In alternative implementations, there may be slight undercompression or overcompression. For example, an undercompression may involve recessing a portion of the surface  332  of the plate  330  coextensive with the coating  320 . An overcompression may involve having that surface portion proud of the portion engaging the surface  72 . The compression may be to a final thickness T 5  which may be coincident with the depth D and similar to the thickness T 1  identified above. 
         [0036]      FIG. 9  shows a lateral perimeter  340  of the recess/trough  300 .  FIG. 9  also shows a perimeter  342  of a baseline coated area of the baseline casting  70 . Hatching shows coating extent rather than sectioning. Whereas much of the perimeter  342  falls along the rotor bores (boundary of rotor lobe sweep), the adjacent perimeter  340  falls slightly therebeyond. 
         [0037]    One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, it might be applied to various compressors including open-drive compressors, single-rotor screw compressors, or other multi-rotor screw compressors. Accordingly, other embodiments are within the scope of the following claims.

Technology Classification (CPC): 8