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 that are used to increase performance, for
instance some have 3 shafts, but I'm not an expert. And no, I don't know how
the 3-shaft versions work! Anyone?



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 can vary over the length of the screws.



--
Peter Fairbrother