Positive displacement pump having piston and/or liner with vapor deposited polymer surface

Abstract

A metering pump for providing relatively small, precise volumes of fluid is provided. The pump piston and/or the interior of the cylinder has a surface finish exhibiting a roughness average within selected limits and a vapor deposited polymer such as polytetrafluoroethylene. A drive mechanism is provided for simultaneously rotating and reciprocating the piston within a cylindrical chamber. Fluid is thereby drawn into the chamber and expelled.

What is claimed is:  
     
         1 . A metering pump for dispensing small, precise volumes of fluid, comprising: 
 a pump housing;    a chamber within said pump housing, said chamber being bounded by a cylindrical wall within said housing;    a piston having a cylindrical body portion extending within said chamber, said cylindrical body portion of said piston having a cylindrical outer surface and a duct formed therein;    two or more passages extending through said pump housing and communicating with said chamber;    a drive mechanism coupled to said piston for rotating and reciprocating said piston within said chamber;    said outer surface of said cylindrical body portion of said piston including a surface finish exhibiting a roughness average of at least about four microinches and a vapor deposited polymer.    
     
     
         2 . A metering pump as described in  claim 1 , wherein said polymer comprises polytetrafluoroethylene.  
     
     
         3 . A metering pump as described in  claim 1 , wherein said polymer consists essentially of polytetrafluoroethylene.  
     
     
         4 . A metering pump as described in  claim 1 , wherein the total diametrical clearance between said cylindrical outer surface of said piston and said cylindrical wall of said chamber is between about 0.000050 and 0.000100 inches.  
     
     
         5 . A metering pump as described in  claim 4 , wherein said surface finish exhibits a roughness average of at least about eight microinches.  
     
     
         6 . A metering pump as described in  claim 5 , wherein said piston is comprised of a ceramic material and said surface finish exhibits a roughness average of between about eight and sixteen microinches.  
     
     
         7 . A metering pump as described in  claim 5 , wherein said cylindrical wall includes a surface finish exhibiting a roughness average of at least about eight microinches.  
     
     
         8 . A metering pump as described in  claim 7 , wherein said cylindrical wall includes a surface finish exhibiting a roughness average of between about eight and sixteen microinches.  
     
     
         9 . A metering pump as described in  claim 7 , including a seal within said pump housing and engaging said cylindrical body portion of said piston.  
     
     
         10 . A metering pump as described in  claim 9 , including a ceramic liner within said housing, said ceramic liner bounding said chamber.  
     
     
         11 . A metering pump for dispensing small, precise volumes of fluid, comprising: 
 a piston including a substantially cylindrical outer surface having a surface finish exhibiting a roughness average of at least about four microinches and a duct formed within said outer surface;    a housing having an inner surface defining a cylindrical chamber, said piston extending into said cylindrical chamber;    an inlet passage extending through said housing;    an outlet passage extending through said housing;    means for rotating said piston about a longitudinal axis;    means for reciprocating said piston within said chamber such that fluid is drawn into said chamber through said inlet passage and expelled through said outlet passage; and    a vapor deposited polymer on said substantially cylindrical outer surface of said piston.    
     
     
         12 . A metering pump as described in  claim 11  wherein said polymer comprises polytetrafluoroethylene.  
     
     
         13 . A metering pump as described in  claim 11  wherein said polymer consists essentially of polytetrafluoroethylene.  
     
     
         14 . A metering pump as described in  claim 11  wherein the total diametrical clearance between said outer surface of said piston and said inner surface of said housing is between about 0.000050 and 0.000100 inches.  
     
     
         15 . A metering pump as described in  claim 14  wherein said surface finish of said piston exhibits a roughness average between about eight and sixteen microinches.  
     
     
         16 . A metering pump as described in  claim 14  wherein said inner surface of said housing exhibits a roughness average of at least about four microinches.  
     
     
         17 . A metering pump as described in  claim 16  wherein said inner surface of said housing and said outer surface of said piston are comprised of ceramic materials.  
     
     
         18 . A piston and cylinder assembly for a metering pump for dispensing small, precise volumes of fluid, comprising: 
 a ceramic piston including a substantially cylindrical outer surface having a surface finish exhibiting a roughness average of at least about four microinches and a duct formed within said surface;    a housing having a ceramic inner surface defining a cylindrical chamber and exhibiting a roughness average of at least about four microinches, said piston extending within said chamber;    a vapor deposited polymer comprising polytetrafluoroethylene on at least one of said substantially cylindrical outer surface of said piston and said inner surface of said housing.    
     
     
         19 . The assembly of  claim 18  wherein the total diametrical clearance between said cylindrical outer surface of said piston and said inner surface of said housing is between about 0.000050 and 0.000100 inches.  
     
     
         20 . The assembly of  claim 18  including a seal mounted to said housing and engaging said cylindrical outer surface of said piston.  
     
     
         21 . The assembly of  claim 18  wherein said surface finish of said piston exhibits a roughness average of at least about eight microinches and said polymer is on said outer surface of said piston.  
     
     
         22 . The assembly of  claim 21  including an inlet passage extending through said housing and an outlet passage extending through said housing.  
     
     
         23 . The assembly of  claim 21  wherein said housing includes a ceramic liner mounted to said housing and defining said ceramic inner surface, and first and second openings in said liner communicating, respectively, with said inlet and outlet passages.  
     
     
         24 . The assembly of  claim 23  wherein said roughness averages of said outer surface of said piston and said ceramic inner surface are less than about sixteen microinches.