The
Ball Piston Pump is a very simple pump design. It has a rotor which revolves around
an internal stator. The rotor has twelve cylinders machined out of it, and each
cylinder has a ball inside which can slide in and out of the cylinder. In
the twelve cylinder model shown here, there are only thirteen moving parts in
the pump -- twelve balls, and the rotor.
From TDC (Top Dead Center) the cylinders
pass over the intake port for 180 degrees, then pass over the outlet port for
180 degrees.
The balls ride along a two-railed track machined into the outer
housing. The balls revolve around the pump in a perfect circle.
Because the
centerpoint of the circle which the balls revolve around is offset from the centerpoint
of the stator and the rotor, the balls and the rotor have relative motion to each
other.
This relative motion increases and decreases the volume of each cylinder, allowing
the mechanism to draw in fluid during one half cycle and expel it during the other
half cycle. As long as the speed is greater than about 100 revolutions per
minute (the exact value depends in part on available suction lift or suction head),
centrifugal force is enough to keep the balls rolling along the track while fluid
is drawn in.
This pump can be remarkably efficient, in part because no parts
of the pump reciprocate (the reciprocal motion in the cylinders is due to the
relative motion of the balls on their circle meshing with the rotor on its circle).
The track the balls revolve around is a dual track. The primary purpose of
this track is to maintain the ball in constant radial alignment within its cylinder.
It also serves to keep the balls from slipping along the outer housing.
The
motion can be perceived as the ball moving relative to the cylinder, or the cylinder
moving relative to the ball.
The two sides of the track (each of which is a circle) get further and further
apart the slower the ball is going (nearest TDC). Except at one point in the cycle
(Bottom Dead Center), the outermost edge of the ball is in a void and touches
nothing. The rate of spin of each ball remains constant, but since its "orbital
speed" varies with its distance from the centerpoint of the rotor's circle --
NOT the distance from the centerpoint of the circle inscribed by the balls --
the actual speed of each ball varies. They are going slowest at TDC and fastest
at BDC.
The track causes the ball to speed up (as the track comes together)
and slow down (as the track separates) in exact proportion to the amount of speed
change required to keep the ball in radial alignment within the cylinder. Ergo:
Extremely low friction!
This pump is in the experimental stage as this is
being written. This design and several similar ones are also being developed for
use as engines and compressors.
(Note: This entry is also available in animated format
at the Ball Piston Group's web site (see below).) 
