From: zenadsl6186@zen.co.uk   
      
   Greg  wrote   
      
   > Peter Fairbrother  wrote in message   
      
   >> Quimby expanders are good for tolerance requirements, as the pressure   
   >> difference is split between several seals.   
   >   
   > What is a Quimby expander?   
      
   A screw pump has two shafts with opposite meshing threads in a close-fitting   
   container. The pockets created between the shafts and the container travel   
   lengthwise when the shafts rotate. Liquids or gases caught in the pockets   
   are thereby pumped.   
      
   The various pockets are at different pressures - say the first one is at   
   1500 psi, the second at 1,000 psi, the third at 500 psi and so on; as the   
   seals are between pockets they only have to contain 500 psi each, while the   
   overall pressure is 1500 psi.   
      
   A Quimby screw pump is a double screw pump, with screws at both ends and the   
   inlet in the middle. The screws are handed oppositely at each end so the   
   shafts have no overall endwise force, and the single stuffing box is on a   
   low pressure end.   
      
   There are a few other tricks than are used to increase performance, for   
   instance some have 3 shafts, but I'm not an expert.   
      
      
      
   These are all kind of hard to describe, but   
   http://www.animatedsoftware.com/pumpglos/quimbysc.htm shows a Quimby pump   
   suitable for liquids.   
      
      
   The Quimby expander is similar but the flow drives the shafts rather than   
   the other way round. If used for gases then the OD, depth and pitch of the   
   thread varies over the length of the screws.   
      
      
      
   --   
   Peter Fairbrother   
      
   --- SoupGate-Win32 v1.05   
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