Ball Piston Pump (from Internet Glossary of Pumps)
Ball Piston Pump
(Note: This entry is also available in animated format at the Ball Piston Group's web site (see below).)

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.

For more information about this pump (including our own animations!):
Visit The Ball Piston Engine Group's World Wide Web site:

Go to web page describing Statistics Explained

CD-ROM -- only $59.95!

  • This Glossary is a part of a complete educational tutorial about pumps!
  • Mechanical pumps are the second most common machine in the world (after electric motors).
  • Most people are unfamiliar with how the many different kinds of pumps work!
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Last modified February, 2002
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