Abstract:
A rotary blower rotor includes a rotor body having a corrosion-resistant coating covering the rotor body. An abradable coating covers at least a portion of the corrosion-resistant coating for providing an essentially zero operating clearance for increasing a volumetric efficiency of the rotary blower. A rotary blower including a rotor with a corrosion-resistant coating is also provided.

Description:
FIELD OF THE INVENTION 
     The present invention relates in general to a rotary blower, such as a Roots-type rotary blower, typically used as an automotive supercharger, with an abradable coating for increasing the volumetric efficiency of the rotary blower, and, in particular, to a corrosion-resistant rotary blower rotor having an abradable coating. 
     BACKGROUND OF THE DISCLOSURE 
     Rotary blowers of the Roots type typically include a pair of meshed, lobed rotors having either straight lobes or lobes with a helical twist with each of the rotors being mounted on a shaft, and each shaft having mounted thereon a timing gear. Rotary blowers, particularly Roots blowers are employed as superchargers for internal combustion engines and normally operate at relatively high speeds, typically in the range of 10,000 to 20,000 revolutions per minute (rpm) for transferring large volumes of a compressible fluid like air, but without compressing the air internally within the blower. 
     It is desirable that the rotors mesh with each other, to transfer large volumes of air from an inlet port to a higher pressure at the outlet port. Operating clearances to compensate for thermal expansion and/or bending due to loads are intentionally designed for the movement of the parts so that the rotors actually do not touch each other or the housing. Also, it has been the practice to epoxy coat the rotors such that any inadvertent contact does not result in the galling of the rotors or the housing in which they are contained. The designed operating clearances, even though necessary, limit the efficiency of the rotary blower by allowing leakage. This creation of a leakage path reduces the volumetric efficiency of the rotary blower. 
     One known approach to improving pumping efficiency of a rotary blower is the use of a coating with an abradable material. While known supercharger rotor abradable coatings provide, among other things, increased volumetric efficiency of the rotary blower and sufficient lubricating properties, they have been found to exhibit relatively poor corrosion resistance, limiting their use to supercharger applications in which the supercharger is not be exposed to a corrosive environment. For example, known supercharger abradable coatings are generally incompatible with marine engines that operate in a salt water environment, as the relatively high salt content ambient air may corrode the rotors. 
     BRIEF SUMMARY OF THE INVENTION 
     A rotary blower rotor is disclosed that includes a rotor body having a corrosion-resistant coating covering the rotor body. An abradable coating covers at least a portion of the corrosion-resistant coating for providing an essentially zero operating clearance for increasing a volumetric efficiency of the rotary blower. The corrosion-resistant coating inhibits corrosion of the rotor body during exposure to a corrosive environment. 
     In an embodiment of the present invention, the corrosion-resistant coating comprises an electrolytic ceramic coating that exhibits excellent resistance to various corrosive environments, and forms a foundation exhibiting excellent adhesion to the abradable coating. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side elevation view of an exemplary Roots-type rotary blower of the type with which the present invention may be utilized; 
         FIG. 2  is a cross-sectional view of the exemplary Roots-type rotary blower of  FIG. 1 , showing a pair of rotors according to an embodiment of the present invention; 
         FIG. 3  is a cross-sectional view of a rotor shown in  FIG. 2 ; 
         FIG. 4  is a photograph of a rotor according to an embodiment of the present invention shown after an ASTM-B117 salt spray test; and 
         FIG. 5  is a photograph of a prior art rotor having only an abradable coating shown after an ASTM-B117 salt spray test. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, which are not intended to limit the present invention, and first in particular to  FIGS. 1 and 2 , there is shown an exemplary rotary pump or blower of the Roots type, generally designated  11 . Rotary blower  11  may be better understood by reference to U.S. Pat. Nos. 4,828,467; 5,118,268; and 5,320,508, all of which are assigned to the Assignee of the present invention and hereby incorporated by reference. 
     As is well known in the art, rotary blowers are used typically to pump or transfer volumes of a compressible fluid such as air from an inlet port opening to an outlet port opening without compressing the air in the transfer volumes prior to exposing it to higher pressure air at the outlet opening. Rotary blower  11  comprises a housing assembly  13  which includes a main housing member  15 , bearing plate  17 , and the drive housing member  19 . The three members are secured together by a plurality of fasteners  21 . 
     Referring next to  FIG. 2 , the main housing member  15  is a unitary member defining cylindrical wall surfaces  23 ,  25  which define parallel transverse overlapping cylindrical chambers  27  and  29 , respectively. Chambers  27 ,  29  have rotor-shaft subassemblies  31 ,  33 , respectively mounted therein for counter-rotation, with axes substantially coincident with the respective axes of the blower  11  as is known in this art. Subassembly  31  has a helical twist in a counterclockwise direction as indicated by the arrow adjacent reference numeral  31  in  FIG. 2 . The subassembly  33  has a helical twist in the clockwise direction as shown by the arrow adjacent reference numeral  39  in  FIG. 2 . For purposes of explaining the use of the corrosion-resistant coating and abradable coating in accordance with the present invention, the subassemblies  31  and  33  will be considered identical, and only one will be described in reference to the use of the coatings hereinafter. 
     Referring also to  FIG. 3 , there is shown a cross-sectional view of a rotor  39 . Rotor  39  comprises a body  40  having three separate lobes  43 ,  45 , and  47  which connect together, or preferably are formed integrally, to define a generally cylindrical web portion  49 . A shaft  37 ,  41  is disposed within a central bore portion  51 . Each of the lobes  43 ,  45 , and  47  may define hollow chambers  53 ,  55 ,  57 , respectively therein, although the present invention is equally applicable to both solid and hollow rotors. 
     To facilitate a better understanding of the structure in accordance with the present invention and for ease of illustration  FIG. 3  depicts rotor  39  as a straight lobed rotor. It should be understood that the present invention is equally applicable to any shaped rotor whether it is helical or straight lobed. 
     In  FIG. 3 , there is shown an abradable coating  61  preferably covering the entire outer surface of rotor  39 . Coating  61  may include a mixture of a coating material base or matrix which is preferably an epoxy polymer resin matrix in powder form and a solid lubricant. Exemplary coatings  61  are described in U.S. Pat. No. 6,688,867, which is owned by the Assignee of the present invention and incorporated by reference herein in its entirety. 
     Referring still to  FIG. 3 , a corrosion-resistant coating  63  is disposed between the rotor  31  and the abradable coating  61 . In an embodiment of the present invention, corrosion-resistant coating  63  is an electrolytic ceramic material, such as the electrolytic titanium ceramic coating Alodine® marketed by Henkel KGaA. The corrosion-resistant coating  63  may be deposited over the rotor  31  at a controlled thickness of approximately 5-7 microns (μm) with a tolerance of less than +/−0.5 microns (μm). The corrosion-resistant coating  63  may be applied with an electrostatic or air atomized spray process, but may also be applied with a liquid process such as a liquid spraying or immersion process. The adhesion of the corrosion-resistant coating  63  on the rotor surface may be improved with surface preparation of the substrate by mechanical means such as machining, sanding, grit blasting or the like, or alternatively with chemical means for surface treatment such as etching, degreasing, solvent cleaning or chemical treatment such as an alkaline or phosphate wash. 
     It is desirable for the corrosion-resistant coating  63  to maintain its structure without peeling at contact areas, and to have good adhesion to aluminum or other lightweight metals employed in the rotor  39 . Also, the corrosion-resistant coating  63  should not be harmful to the catalytic converter or the heat exhaust gas oxygen (HEGO) sensor if any particles become entrained into the engine after the break-in period. As such, the corrosion-resistant coating  63  particles do need to be combustible. In addition, the corrosion-resistant coating  63  also has compatibility with gasoline, oil, water (including salt water), alcohol, exhaust gas, and synthetic lubricating oils. 
     In the development of the blower which uses the corrosion-resistant coating material of the present invention, a variety of coating materials were investigated. Table 1 lists the results of several of these coating materials. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Corrosion-Resistant Coating Materials 
               
             
          
           
               
                   
                 Abradable 
                 Titanium Ceramic 
                   
               
               
                   
                 Coating Only 
                 Coating 
                 Teflon 
               
               
                   
                   
               
             
          
           
               
                 Nominal Thickness 
                 80–130 μm 
                 5–7 μm 
                 40–60 μm 
               
               
                 Operating Temperature 
                 −40° to 
                 −40° to 
                 −40° to 
               
               
                   
                 150° C. 
                 600+° C. 
                 150° C. 
               
               
                 Cure Time/Temp. 
                 Approx. 20 
                 Approx. 1.5 min/ 
                 Approx. 20 
               
               
                   
                 min/200° C. 
                 Room Temp. 
                 min/373° C. 
               
               
                 Adhesion to Rotor 
                 Very Good 
                 Very Good 
                 Okay 
               
               
                 Adhesion to Abradable 
                 N/A 
                 Excellent 
                 Poor 
               
               
                 Coating 
               
               
                 ASTM-B117 Salt- 
                 Failed* 
                 Passed** 
                 Passed 
               
               
                 Spray Test 
               
               
                   
               
               
                 *Photograph of ASTM-B117 test results shown in FIG. 5. 
               
               
                 **Photograph of ASTM-B117 test results shown in FIG. 4. 
               
             
          
         
       
     
     The abradable coating  61  is deposited over the corrosion-resistant coating  63  so that the abradable coating  61  and the corrosion-resistant coating  63  have a collective thickness ranging from about 80 microns (μm) to about 130 (μm). The coated rotors can have clearances due to manufacturing tolerances that may range from rotor to rotor from about 0 mils to about 7 mils, and rotor to housing that may range from about 0 mils to about 3 mils. Preferably, the thickness of the abradable coating material on the rotors is such that there is a slight interference fit between the rotors and the housing. During the assembly process, the rotary blower is operated on line for a brief break-in period. The term “break-in” as used herein is intended to refer to an operation cycle which lasts as a minimum approximately two minutes where the rotary blower undergoes a ramp from about 2000 rpm to about 16,000 rpm, and then back down. Of course, the break-in period can include but is not limited to any operation cycle employed to abrade the coating to an essentially zero operating clearance. 
     The invention has been described in great detail in the foregoing specification, and it is believed that various alterations and modifications of the invention will become apparent to those skilled in the art from a reading and understanding of the specification. It is intended that all such alterations and modifications are included in the invention, insofar as they come within the scope of the appended claims.