Sean Breheny wrote:
> Isn't the screw, when stopped, a continuous inclined plane so that
> water poured in at one end would just run "downhill" around the turns
> of the screw and out the other end?

No.  Go get the biggest nut for a machine screw you can get your hands on
easily.  Dunk it in water and pull it out such that the screw axis is
horizontal.  Now look at the threads inside and note how water is trapped in
each thread separately at the bottom.  Now rotate the nut and you can see
the trapped puddles of water stay at the bottom but move along the screw
axis.  In fact, they are acting like the screw inside the nut.

Now you can tilt the nut so that one end of the screw axis is a little
higher than the other.  Some water is still trapped in each thread, although
a little less.  By rotating the nut, you can now "pump" water up hill by
turning it one way and downhill by turning it the other way.  If you tilt
the nut all the way so that the screw axis is vertical, it no longer holds
any water.

Archemedes spirals use this principal, with the thread shape specifically
designed to hold liquid.  Their tilt is usually around 45 degrees.  That may
be too much for the nut since its thread shape was designed to mate with a
machine screw, not to hold water.


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